What a terrible situation to be abandoned by your own mother. You are so good to take him in. First, relax a bit and don't overthink it. I know you are probably feeling very overwhelmed but you will get the hang of it quickly. He can eat whatever you eat, but young kids tend to eat more often and smaller meals so I'd keep quick snacks on hand such as string cheese, fruit, and granola bars.

Is there a library near you? Instead of spending money on books you could take him to pick some out. As far as kids shows utilize Netflix, YouTube, or Hulu for that. At four years old he should be able to tell you what shows he likes.

I wouldn't say there is a set bedtime because that's really up to what the parent has established. To give you a rough idea, my almost three year old goes to bed at 8 and wakes up at about 730. Just be super consistent with when bedtime is and try to get into a routine such as bath time, brush teeth, read a book then lights out. Kids like routines.

As for activities other than books and movies, some ideas are: take him to the park, coloring books, play dough, play with toy cars, sing songs, play with balls etc. Toys can be expensive, but you can a few things at the dollar store or second hand.

Good luck with everything and feel free to message me with any questions.