Hi,

Having upgraded my 4670 for a 4850 I can not run this game maxxed out at 100fps+
which is great but I'm totally stuck and have been for over a week.

Level 4, I'm dying all the time, right at the start of the level, you walk in and the door closes (can't use the 'put rock on pressure plate trick') and those poisonous mushroom things kill me very fast. No matter what I do, I die.

Help / Advice / Cheat?

What's your party composition?

That's true. I hit them as they are entering the square, then move.
And here's advice that goes with a lot of situations: don't let them pin you in a corner! You're in bad shape if you're completely bodyblocked. Just keep an eye on where they are and keep moving.

2 warriors up front, mage and rogue behind.

If your rogue is specialized in ranged attacks then make heavy use of that during this fight. Load up a ranged attack on your mage before entering the room, but after you fire that off don't use the mage for the rest of the fight (trying to enter spells will just distract you). If your mage knows poison shield then that will make this battle much easier. The main difficulty with this fight is, as others have mentioned, that the large mushrooms fire off an AoE poison attack whenever they are facing you from an adjacent square, and this can very quickly kill of your party if you get hit by several of them. Thus try to do as much ranged damage as you can, and if you need to go up close and personal always make sure you're attacking from the side or the back, then quickly move away before they can turn to face you. And again, as has been mentioned, make sure you don't get pinned in a corner. This is the first big fight in the game where you'll really need to move both quickly and carefully, but the kinds of tactics you'll be employing are important for the rest of the game.