Just use JPG as you want, but add a final screen shader that restricts the game's palette to only the 4 colors you want, rounding each one up to its nearest one. That way you can save yourself the space while ensuring it's always 4 colors!
I like it, and if I were doing Unity WebGL I would... but I'm learning Phaser by doing this too! So I'm just going to keep to PNG, I managed to get the size under control by being smarter about it lol... ty for reply
