This video should help you get started.
Did you look at the custom tutorials thread pinned in the board? That will help more as well. Many methods used on older gen ponies can be used on G4.
As for sanding, I'll hope someone else has the answer for that. Not within my realm of knowledge.
I agree with checking out the pinned tutorials here, I’m just teaching myself to customize and they have been so helpful!
As for buying hair, I’m pretty sure all the major suppliers I know of only sell by the color? (If someone does sell big bulks of multiple colors I’m interested lol!) I buy my hair from retrodolls and they’re great! There are soooo many colors to choose from, it really takes me forever when I go shopping! The hair descriptions will also tell you how big the hank you’re buying is and how many Hanks it takes to rehair certain toys, of course including ponies.
I bought my reroot tool there too!
I’m not an expert by ANY means, but if you want to know what NOT to do I can definitely give you a few pointers on what went wrong with my first and how to recover!
I’ve got more hair on the way to try some more though! It’s fun once you get the hang of it!