Hi Dreamingofcakeallnight:
Well for a 4 year old you may have stumbled across Oxford Owl (both for Reading & Maths) support in first school years: www.oxfordowl.co.uk/ There is a lot of advice and ideas for parents and some games for children that should be age appropriate (but I'd try them out first to judge if they're too hard or not).
Obvious things to do are snakes & ladders - play forward for addition and backwards for subtraction.
Evens and Odds: Walk up one side of the street (maybe on the way to the shops or the park) and back the other side. Start talking about the differences in the numbers.
Puzzles (not just jigsaw but wooden shapes that fit together in a certain way) are also great for working on maths ability. Tangram puzzles (where you have set shapes but are tasked with making different things (i.e. animals or objects) using the shapes. There are usually two ways to play this: each - shows you where the shapes are repositioned with outlines or the hard way - new shape, but no outlines, so you have to work out the arrangement (probably too advanced for age 4 - but who knows - this is MN).
Blocks: working with building blocks and discussing the names of various shapes. (working with concepts of 2D & 3D).
Cooking (measurements): let your DC help you cook by measuring out all the ingredients. Familiarity with the relative size of things and equivalence of things (i.e. 1 teaspoon = 5 ml) is really useful, but usually is learned from familiarity - so repeatedly working with these things).
count/ record: If you are around a place a lot and your DC is wild about something (say dragon flies) - you could record how many you see each time you visit and compare it with last time. When do you see a lot - on calm warm days? Windy, wet days? When do numbers drop off?
And relax.....
If you DC is already adding/ subtracting at 4 he's streets ahead of most kids and your problem (and the teachers - if he's going to a state school) will be entertaining him and encouraging this. Try to find things he enjoys which include a maths dimension and you'll be winning.
It isn't a sprint this education thing - it's a marathon. So one of the best ways of supporting maths or English is to find things that relate to it but in unusual ways (going to a museum and learning about science, going to a natural history museum and learning about fossils, going to a historic house of a famous author, going to a play or film of a favourite children's author/ story, etc...
HTH