In Sandbox Mode, Kerbals don't die. They "mysteriously disappear".
They respawn after a short while (the original three (Jebediah, Bill, Bob) at a quicker rate than generic Kerbals), and can then be sent to their doom again. And again. And again.
Your question is actually about how fast you are going to spend the fuel of your first stage: Are you using four or five engines simultaneously to convert it into speed? When you use all 5 engines but throttle down to 80% thrust, it's exactly the same as if you would use 4 engines at 100% thrust. To answer this question, we first need to talk about what forces affect a rocket during lift-off.
There are two forces which prevent your rocket from getting into orbit.
- Gravity
- Atmospheric Drag
The first, gravity, is accelerating your rocket constantly downward until you have enough horizontal velocity to cancel it (achieved a stable orbit). The more time you spend affected by gravity, the more speed ("delta-v") does the gravity give you which you have to cancel by expending additional fuel. For that reason it is economically to accelerate early, so you get to your orbit faster and consume less acceleration from the gravity.
But there is also the second force: atmospheric drag. Atmospheric drag depends on atmospheric pressure and speed. The relationship with speed is quadratic. The faster you go, the more speed ("delta-V") you lose through air friction. That means it might not be so good after all to go too fast while you are still in the lower atmosphere.
So where do these two factors cancel out?
The ideal speed to balance atmospheric drag and gravity drag (assuming perfectly vertical ascent) is equal to the terminal velocity on the current atmospheric density. That speed depends on how aerodynamic your vehicle is.
When you go faster than this, you are wasting fuel on atmospheric drag. When you go slower than this, you are spending unnecessary fuel to fight gravity.
To get back to your initial question "what's better: serial staging or parallel crossfeed staging": It depends on your total thrust-to-weight ratio in the lower atmosphere. But my general experience is that a rocket gets higher with cross-feeding.
But what when you are already in orbit?
The truth is, it doesn't matter. The amount of delta-v you get per liter of fuel depends on the total weight of the ship and the average fuel-efficiency (Isp) of the engines you use. It doesn't matter how fast (through how many engines) you spend it. All that matters is to avoid having more mass than necessary (get rid of fuel tanks when they are empty). An orbital transfer stage with less engine power is more efficient, only because it tends to have a lower overall mass. This, however, is bought with longer burn-times to get the same amount of delta-v. Longer burn-times can sometimes mean less efficient burns because you can't hit your maneuver nodes that exactly, but this only matters in situations where a burn is very time-critical.
Best Answer
You're right, that setup wont get you to Gilly and back, for a variety of reasons.
Here are four ways you can improve your game and make for yourself a rocket that will perform that mission. I'm assuming, since you're playing KSP in the first place, that you want the achievement of having made and flown the entire thing, not just downloaded someone's .craft file!
* Caterpillar Staging is when you have engines that aren't jettisoned with the fuel tanks, as pictured below. (The parts with the diagonal lines are meant to represent the 2m Rockomax decouplers.) Just like Asparagus Staging, Caterpillar Staging is meant to drop the dry weight of empty tanks and therefore keep the rest of the craft light and long range.