4mm?
That's very thick isn't it?
I think I would put a couple of lengths of angle iron in a vice lined up perfectly with the ally sandwiched between them, then as you say bash it with a hammer. i have done this before with both ally and thin steel, not having a proper metal bender.
There may even be a way of using the two pieces of angle spooned with the sheet ally in between and then pressing them together in the vice, but i have not tried it.
I believe the way of annealing ally is to rub it with soap then heat it with a blowtorch till it turns brown.
But I think you'd be better off asking a trained man like
@My Old Landy
or
@DanClarke .
I will watch this with interest.
But I can't help thinking a thin piece of ally is easier to work with.
I have had to make all sorts of stupid complex shapes with ally before, for instance a foot rest for my left foot to go in a cut out I made for my kit car, that wasn't easy and then I changed the gear box and made it even harder for myself.
and what is feckin annoying it that unlike steel, if you make a mistake you cannot just weld a new bit in as you would with steel.