For me 4 months is too early to wean so look for bigger (older) ones when selecting, don't go for the "poor wee one" as you may have problems with nutrition, worms etc on top of having a weanling unused to handling, transport etc and stress is inevitable so watch for early colic too..
Feedwise adlib hay, a mineral lick (not just salt) and if in poor condition make it a molassed bucket mineral lick and enjoy the sticky noses.. Stud or youngstock mix will be far too rich, you won't just get long legs but laminitis risk so leave it all and at most give something like alfalfa, beet pulp or grass nuts, no cereals.
Keep them in together until you can handle them safely, they'll be so distressed and young they'll come round quite quickly for a little time and effort without pressure. Sit in the stable with them and offer to scratch bums and hind legs as a mother would, rather than go for the front end initially. Work gradually up the back and withers over time (maybe one session maybe several). Neck and heads are last - you'll get the noses coming to you soon enough and then don't spoil them with overhandling and set clear firm and consistent boundaries, what may be cute and rewarding in a rescue foal is not in an adult so think what you want the behaviour like long term and gently insist on that. Then if you do need to sell or want a child to be safe around the pony, it isn't confused about what it can and can't do and doesn't have to keep checking by trying..
Best of luck. I worked with an 18 month Welsh A unhandled herd kept colt and got him to leadrein standard, very rewarding and educational experience if you have the patience and skill for it.