I have many friends who want to play Doom with me so I've been thinking playing normal games with 4 or more marines. Has anyone tried it before and did you have to make some new rules for it? Maybe playing with Nightmare difficulty would be good challenge then?

I guess just Nightmare difficulty alone won't do - as there won't be enough weapons to equip all marines adequately. Once they all do have weapons though, Nightmare does sound like it'd make sense.

Looking forward to your experiences

I played as invader with 4-5marine players.We use preparations rules some as for 3marines,but i add additional weapons(chainsaw,shotgun or

machinegun are the best starting weapons) in starting room,according to the number of additional rules.We also have a home rule:if 4 or more marines playing invader allowed to spawn all figures that he has(in case of expansion monsters)

I've tried that and found that with 4 marines things don't get much more difficult for the Invader, but with 5-6 marines it's a cakewalk for them unless they really suck at organizing (I always use the new deck, not the original one). So I usually make larger maps and split them into two groups to make things more interesting .

A scenario that splits them into teams of 3/4 but requires each team to help each other move forward (Switches, etc) would work in this case. Come up with two maps, each having switches that would help the other team forward. Then play in this order:

Team 1 moves

Invader plays only on Team 1's map

Team 2 moves

Invader plays on Teams 2's map

That would keep the invader from being too stretched thin...and also keeps the invader from moving twice on a single team. This would be the simplest way I can think of to get that many people in play without making it too easy on them, or too hard on the invader.