I like watching them grow and develop - learning a bit about child development is interesting here because you get to see it in action. There is a good (old) series of books called "Your X Year Old" by Louise Bates Ames that I like, or look up the old series of Child Of Our Time or that Secret Lives of Four Year Olds programme.
I like talking to them to see what they think about the world - that is always fascinating and sometimes hilarious.
Four year olds are pretty full on and really benefit from being kept busy - do you have a mixture of things planned for each day? I tend to find that if I manage the day a bit like a giant lesson plan (I used to teach EFL) it goes better. So I have morning chill/TV/toys/lazy breakfast time until about 9, then we try to do something either an activity in the house, or go out. If we're home, early lunch around 11/11:30. If out we might be able to push it to about 12/12:30.
After lunch some free play and then around 2pm they get antsy again so snack and then go out for a local walk or another activity. Back by 4 to start on dinner. Put TV on (any time from 3ish) especially if I can't be arsed to deal with offers of "help". DH is usually home by around 5 so that's not too bad.
By activities I mean I have a mixture of:
"Big" toys that I don't necessarily need to have input into but don't come out all the time, so I might set up a play cafe or something or dig some bigger playset out of a pile it's been buried in or put the play tunnels out in the living room.
Board games or jigsaws that I'll join in and play/help but only take about 15 mins.
Colouring books/puzzle books/sticker books, writing or pen or scissor practice and free printable stuff. You can just google "Free paw patrol colouring" or whatever they like and find loads of stuff. This can occupy them for a while especially if you make impressed noises every now and again. Craft glue or glitter glue is a risky favourite.
Occasional kits like a baking kit or craft kit. I find it's worth having around 1 of these per school holiday week.
Me getting on with some household task that I don't mind if they help with, even if their help is "help".
Playdough or a tray of rice, kinetic sand, hama beads etc. Anything fiddly and requiring concentration. Needs supervision but not a huge amount of input unless you want to. I sit with a cup of tea.
Reading books together - especially a more "grown up" book like an Usborne fact book or something. (Library is a great outing).
Also, everything is better with company. Day trips out with friends are great. Even just simple playdates alternating houses or meeting people at the park is great or more everyday outings like swimming. Visit relatives or other people who are also on school holidays.
It's also only enjoyable if you get a break - it can burn you out if you're constant kid wrangling.