Hmm... 4 months old is a little premature don't you think? The normal weaning time is 6 months. Be careful weaning them too early as it could lead to deformity in the foals development. Remember foals get a lot of nutrients and growth hormones from their mothers milk, and cutting them off from that easy digestible nutrition too early could be more detrimental than good. Normally a colts digestion system is still developing so getting them acquainted to solid food will be more difficult.
Personally id call a vet for this one. Once you get him, have the vet give a general once over, making sure that his bones are developing correctly. Then discuss a feeding regime with your vet based on his personal needs. They may recommend a formula supplement, or mashed up grain, or small doses of grass hay. There are plenty of supplements I like to use on youngsters once you get this one eating correctly and well on the way to proper growth and development. For example I personally adore "Calphormin" as a starter. It's a bit pricy but does wonders in aiding the growth and development of a colts skeletal system. It's rich in calcium and magnesium, both minerals a colt needs for proper growth. Second you should feed him a basic vitamin supplement since he'll most likely be in deficit of that. Something simple like "Millennium Gold" helped me when I was rearing my stud. Besides that I wouldn't recommend anything else. You don't want to overwhelm a young horse with outside supplements, when good simple nutrition is all he needs. I can't recommend any brands, because the vet would be the best one to give you that information. Depending on the colts current situation and what it may need nutritionally would dictate what rations of protein to starches, and proportions of hay (I would push for grass though.) he'll need.
Sunny Black is...
Stallion owner, Assistant trainer/instructor, and Friesian Lover ~ <3