Again, there's no easy way to do that.
You might be familiar with this assumption and I can tell you where it steams off too. D&D 4th edition is quite a different game with some similarities that were kept to brand it as "D&D".
The main problem here is that monsters only have powers they can actually use during an encounter. No more 1st level spells on your epic dragon. No more "the monster has this spells provided it has the time to buff". Everything in 4e happens during combat. And 4e Monsters don't have to deal with save-or-die spells, while most solos have to deal with multiple stuns (which didn't happen in 3.x).
AoO work in a different way, ranged attacks deal damage based on Dex, spells deal bonus damage based not on the level but on the mental scores. 5-feet step is now a movement action (except for kobolds)... Different game.
The easiest thing you can do is to take existing D&D 3.5 monsters or NPCs and present them to the group as if they were the kind of interesting monsters you want to portray.
Maybe choose something that has similar mechanics if you can find it.
In other word(s), reflavor.
First off, I'll quote some general advice about converting settings, which is in the GM section of Savage Worlds Deluxe:
Like an original setting, identify the themes of the other game and
try to adapt with a very few key world rules. Literal translations of
game mechanics from other systems usually just result in cumbersome
sub-systems that don't add one minute of fun to the Savage version.
I think that this advice also applies to converting monsters and other elements from video games. Fortunately, you seem to have the right mindset by considering their place within the setting and describing them as "stronger" or "weaker" rather than pulling out the mechanical numbers from the game.
As you noted, a d6 is usually the baseline for an "average" person in the setting (with "average" generally referring to an average trained person. An average soldier in the military would have d6 Shooting, an average writer who spends his days in a coffee shop will not). Everything else should be in relation to that.
If a Necromorph is noticeably stronger than an average human in the setting, give them a d8 Strength, which will put it on the level with someone who has gone above and beyond average strength training. Giving them a d10 Strength would make it only an equal match for those who are even more dedicated to it and so forth. Put another way, the higher their Strength, the more people they will be overpowering by a greater degree, and the fewer the characters who will be equally matched against it.
You specifically mentioned Smarts as a point of contention. One thing you can do is add the animal intelligence modifier, written as "Smarts d6 (A)". From the book:
Note that for some creatures, Smarts is listed relative to the animal world, and is thus followed by an (A) to remind you that this is animal intelligence, not people intelligence, so don't expect a dolphin to drive off in your tank just because it's a relatively smart animal.
It sounds to me like this might be appropriate for the Necromorphs, since you wrote that they aren't intelligent, but can use hunting tactics to great effect (sounds like they're using the "Smarts trick" combat maneuver). So they might have Smarts d6 (A) or even Smarts d8 (A) if they're very clever. For reference, enemies in the SWD bestiary like a Wolf, Giant Worm, and Lion have Smarts d6 (A). By the way, Attributes cannot go below d4, so you can't have Smarts d4-2 like you suggested.
Hindrances and Edges are generally for intelligent beings rather than monsters. There are some exceptions, such as monsters in the SWD bestiary getting things like Frenzy, but generally these are few and far between and almost always combat related (it doesn't make sense for a zombie to have the Arrogant Hindrance, for instance). Also note that if they do get them, they are usually listed under the "Special Abilities" section of their statblock, rather than an "Edges" or "Hindrances" section. This is more for concision in the statblock than anything else.
Finally, don't forget Special Abilities. Immediately before the bestiary in SWD is a list of monstrous abilities that are generally inappropriate for players, but are important to many monster concepts, such as ethereal and infravision. If you need something that isn't covered by anything else, this is usually where you put it (for instance, the wolf's "go for the throat" ability and the deal about a vampire needing to be invited into a house are new special abilities). However, less is more and you should avoid creating new special abilities unless it is absolutely essential to the concept.
If you'd like to see an example of converting video game enemies into Savage Worlds, check out my Elder Scrolls conversion, complete with a bestiary statting enemies from the games.
Best Answer
Conversion is hard.
Converting characters/monsters between editions is hard. Converting between 4e and 5e is especially hard. One of the primary focuses of 4e, if not the the primary focus, was tactical combat, so 4e has a lot of special mechanics to support that style of play. Because 5e sort of supports tactical combat but certainly doesn't emphasize it, 5e lacks equivalents of a lot of 4e's mechanics.
Any 4e monster is going to have a lot of abilities that can't be easily translated to 5e because the corresponding mechanics simply aren't there, or have changed radically. For example, hit points and AC/attack bonuses scale very differently between the two editions, and that's without getting into harder things like changes to conditions or saving throws.
Try adapting an existing monster instead.
Creating a new homebrew monster can also be pretty difficult, particularly for GMs who don't have a lot of experience with the system. It's much easier, instead, to find an existing published monster that is mechanically a good fit, and simply describe it as the monster you want. For example, you could find the stats for an owlbear and instead describe it as an armored knight, talking about his sword when you roll claw attacks, etc. You can still tweak the mechanics as needed of course, but at that point you're making minor adjustments to something that someone who was more familiar with the system built, rather than trying to built it from scratch yourself, which can be overwhelming.