Jeez...I have 21 on my board right now..this is going to be hard. I guess since this is a hypothetical world I can choose pedals I dont necessarily own yet.
Empress compressor ( dont own one yet but I plan to get one this year)
DR Scientist The Elements ( my favorite dirt pedal it does it all and will never leave my board)
Eventide pitchfactor ( essential to alot of the stuff I like to do with octaves and pitch shifting it can also do digital delay stuff really well)
Hungry Robot "The Wash" ( not out yet but apparently they are going for the multiple stacked delays and reverbs sound in a single pedal format, thats what I would need if limited to 4 pedals. I've heard clips and its sounds really good) or possibly maybe just use my trusty digitech timebender
Ps: A volume pedal for swells is pretty essential too but I guess I would just have to get my guitar volume pinky swell game on point
koko boost because it has two boost modes, el capistan because its the best delay ever plus it has a wah and the repeats can sound like they have chorus on them. wah and probably the strymon flint because it has reverb and modulation!