We spent an entire two days with our Thomas-mad DCs in Thomasland for the most part, with brief forays to nearby rides and the zoo. It was busy but we still rode everything in Thomasland twice by the end of it.
Drayton Manor Park isn't huge, but there should still be a fair few rides for your 4yo to do.
All of the food in and around Thomasland is of the hotdog/nugget/burger variety, but it is possible to get other food at the cafe at the other end of the Thomas line (Mrs Kyndley's IIRC) and there is a big restaurant the other side of the "castle", towards the Ben 10 ride, which has jacket potatoes etc.
FWIW, our 4yo loves the Ben 10 ride too although the queues can get enormous.
LouisaJF the featured engines are Thomas/Percy/Rosie (two operate the line, the other will be sat trackside), Bertie, Lady, Winston, Cranky, Bulstrode, Terence, the Troublesome Trucks and Spencer. There are a selection of Diesels (Salty, Mavis, Dennis, Diesel that I can remember) and the Blue Mountain Quarry engines (Rheneas, Sir Handel, Peter Sam and Skarloey) on their own rides. Oh, and Jeremy the Jet Plane and Harold each have their own rides. James is trackside and can be climbed on. Spencer is in the playground. The other favourites such as Toby, Edward, Henry, Gordon feature as faces around the "engine shed" around Harold's ride, so your child can still pose for photos with them. Emily has a soft play area.
I've probably forgotten someone, let me know which engines your child will want to see!