hi im new to welding, have just purchased a wolf weld 135amp mig welder.
i just wanted to know if it was possible to weld 3mm or 4mm box section and flat plate or will i need a larger welder?
You are about at the machines limits with 4mm, although if you prep you should manage, your other problem is the MIG itself, they are not really known for being a great machine, however I am sure some people are doing ok with them... try some runs on some scrap first and see how it goes.