[RPG] A player made a 4th-level character for our new game and didn’t tell me. Now what

I started to GM 5th edition D&D a few weeks back; I am running them through the Lost Mine of Phandelver adventure.

After a few games I've almost already killed two new level 1 characters but I was intrigued about how the 3rd player's character didn't seem to be fazed. I mentioned in passing, "Wow, you should be pretty close to dying!" (having been hit by three goblin-arrows), to which he replied: "No, I am at 44 health."

Then I asked his level and he replied, "Four."

He never mentioned at all that his character was level 4. He helped everyone else (including me before I decided to GM) make a new level 1 character. The module is for level 1 characters. He wanted to play this particular module because the specific character he is playing has never completed the module.

I am at a loss for what I should do. I considered a few options:

• Find a way to activate a spell that curses him to level 1.
  • Since I am so new to GMing and 5e, I am unsure if that is a viable option.
• I also considered just making everyone else level 4 and having him help level everyone else up for the session.

• Raise the monsters levels for the rest of the adventure.

  • But, I'm sure the level 1 characters would die from the extra damage and I would have to pore over the DMG, which I haven't bought yet. (I'm using the DMing info in the Starter Set.)

• What if anything can I do here?

• Should I raise the monsters' levels to keep it fair and challenging, or just overlook the 'oversight'?
• Should I require the person to stop what we are doing (in the middle of the goblin cave), and force him to make a new level 1 character and reintroduce his character to the group?

Any help would be appreciated.

I am also going to ask other GMs I have played with in the past for any ideas. I will look over any answers I get here after the game session. Thank you all for your help.

(For reference, I started playing D&D in 2nd edition, up to and including Pathfinder, 3e, 3.5, and various other games. I did not play D&D 4e.)

To answer a few questions that your various responses left for me:

• It was his idea to start running a game.
• There are 4 of us [originally 6 but one had to drop out and the 5th
  had been stuck in a 'working on Friday' loop, leaving 4 [3 plus GM].
  He [lets call him 4B] gave the idea of everyone making 1st level
  characters [where my assumption that he was level one to begin with]
• I do not believe 4B was trying to pull a fast one on me, or trying to
  cheat. I have not been able to get a copy of his character sheet to
  look over anything. We used just the core book, no other books [PHB].
  We used backgrounds [there may be some bonuses coming from there].

As for solution options, I have strongly considered either leaving things as they lie and have the goblins target the big buff fighter type that cant see in the dark, or asking him to revert his character to level one. Since 4B is hosting D&D at his house, I worry about how he may react to this. I have considered hosting the games at my house. I like the idea that he gets a visit from a Godly source, but i am unable to get an answer from him on his characters beliefs.

I have already taken steps to possibly avoid any version of him using player knowledge through his character. [Mind you, I am not saying that he is doing this in any way; he has told me he would not do that and has so far shown it through his character's actions that he 'doesn't know' anything.]

One step was to change some of the details of the rooms:

[An example is the room with 3 wolves tied to stakes – the back room connected to another room; I have changed it to that area being fully cut off in rubble, so there is no connection.] I have changed a bit on how the goblins react vs. how the book had them react. [The 2 goblin guards were actually guarding instead of being bored, and almost surprised the characters.]