What are some of the most useful 3 and 4-effect potions and poisons? Im currently experimenting and I only know of a few that I like to use. The Wiki says there's more than what it has listed.

http://www.uesp.net/wiki/Skyrim:Useful_Potions

Not sure if this is the wiki you were referring to or another Elder Scrolls one, but this seems to be generally all-encompassing from my experience with Skyrim Alchemy.

For a beneficial potion with 4 effects from 3 ingredients:
Charred Skeever Hide
Mudcrab Chitin
Wheat - or other Restore Health effect =

Resist Poison/Restore Health/Restore Stamina/Cure Disease.

I'll have to check my notes....there was one I created using slaugterfish eggs, human flesh and a couple other items that, if I recall correctly, did something like -100% magicka regen for 22 seconds, -X magicka or something. It was pretty darn sweet. I found the recipe on the internets, so it's out there.

If your looking for the most valuable potion for selling and leveling, try this.

Either Hanging Moss or Wheat combined with Giants Toe and Creep Cluster. You won't be able to create a more valuable potion than this.

I keep a stack of potions hotkeyed for when I am completely absorbed in an encounter and want an insta-heal that will save me if I need it, without me having to swap out hands, or pause gameplay. The recipe I've found gives the best instant reprieve is:

Bear Claws
Wheat
Eye of Sabre Cat

For: Fortify Health/Restore Stamina/Restore Health

This is the most useful potion to me, and at end game, after I've smithed and enchanted what I need it's the only continual use I get out of the 7 perk points I spent in the Alchemy tree.

Try Fly Amnita + Hawk Beak + Snowberries for a resist shock, frost and fire potion. Great for dragons and encounters vs multiple mages or harpies. (iirc gives 110% protection with Alchemy perks + enchanted gear)
For mages, Glowing Mushroom + Red Mountain Flower + Ectoplasm will give you Fortify Destruction, Fortify Magicka and Magicka Regen.

My favourite 3-effect potion:

Snowberries + Mudcrab Chitin + Purple Mountain Flower

It creates a potion that increases the resistance against fire and frost and restores a bit of stamina. It is most useful for fighting a dragon as a warrior and while the ingredients are pretty common.

Yeah, those are the ones I already know about. Very powerful.