need to help my 4 year old with his talents

Lessons might be a way forward but the structure might be a bit of a problem. Try him with a few first.

If I could ask does your son have a talent for hearing if things are in key? There is a lot of evidence that some people with ASD have a thing called absolute pitch. They can play in key without much training. Another plus is that he can learn maths and music simultaneously. Playing music all entails maths. 

It might be good to get him a durable cheaper keyboard like a Yamaha PSR F51 it's around £70 and has over 100 sounds, a drum machine built in and built in speakers plus a headphone socket. It's built well too. I had a mess around on one a few months ago. I think you'd get a few years out of it and when he gets a bit older it would still be a useful instrument. Some of the Casio synths are good too and cheap. I do like what I saw of the Yamaha though!

There is also a thing coming out called a BlipBox that is basically a modular synth and a drum machine for kids. It looks like a toy but has some features that he would learn from. He would learn to program a synth rather than play conventionally. However it wouldn't have the same longevity and I would say that it could make some random sounds that might be a little irritating for yourself and could scare him.

I'd say making electronic music is a bit of a lifesaver. I can get lost in it all.

I'd say if he's getting creative let him go wild! If he has an interest encourage it, a lot of parents don't realise how theraputic it can be. I've got to say I like your way of thinking and looking for something he can continuously develop and have fun!

For the maths we got the key stage maths books for our children they loved doing the exercises in them and all took their GCSE maths very early because they had taught themselves 

Thank you very much for the reply.

Thank you very much for the detailed advice.

Thank you for the tip

My 3 year old is currently learning to read music using an iPad app. It's surprisingly effective. Teaches them to read music and identify instruments by ear. No human interaction or verbal input required.

Musilla, it's called. 

For music, try to enroll in a music class for young children. See if he takes to a particular instrument. There are some instruments that have miniature versions (e.g. violin) for kids that age, but are still fully capable as musical instruments. If you don't own a piano, it might be good to have an electronic keyboard available. If he has a natural talent in music, he may well be able to learn how to play several instruments over the course of his development. When the time comes, make sure that when you get him a music teacher, they are familiar with and sensitive to his condition. A good, compatible, music teacher could do wonders for his self confidence and growth, but a bad teacher (meaning they aren't sensitive to the way he learns and perhaps overly critical) could turn him off music forever, so be sure to sit in on lessons (which you may have to do anyway, given his age). When he is old enough, playing in a music group with his peers can be a very positive way for him to develop social skills without overwhelming him.

For maths, try to find toys to develop that talent. Construction sets may be particularly fun for him. Consider toys that are designed for older children, as long as you are sure he is past the stage of putting things in his mouth.

I'm not sure how you feel about electronic devices, but there are apps for developing mahematical skills (though these will likely develop quite well on their own just from his observations of how the world works), and there are also apps for language skills so that he can catch up in that regard.

I'm answering not as a parent but from personal experience of having those particular talents as a child.