re: Quantities and ingredients for a stovetop shrimp boil for 4?

Unlike crawfish where the rule of thumb is 5 lbs. per person, shrimp are a lot less needed. My general rule of thumb is 1 1/2 lbs. per person if you are planning on boiling potatoes, corn, mushrooms, sausage, etc. with the shrimp.

Everything has to be damn near finished cooking and soaking when you dump the shrimp in the pot since they will only take a few minutes to cook.

ETA: As far as seasoning, that is an individual thing but I like to use the powdered seafood boil like Louisiana Seafood Mix. To that I'll add some cut up onion, garlic, celery, lemons and bay leaves and let that boil for a while to meld the flavors before adding anything else.