Honestly, for most pesticides, "more isn't better" -- you're just wasting it. The only real exceptions that I can think of to this rule is that sometimes, 1) it's helpful to paint trunks/stubs of trees and shrubs you want to kill, kill for sure, and kill quickly with either a very strong solution of an herbicide, or even the straight concentrate right out of the bottle; or 2) if I HAVE to spray, but rain, especially heavy rain, is forecast within 24 hours, I will do 1 1/2 times the normal rate, hoping that it will help ensure some remains. Even that is wrong, really, certainly NOT an approved method, but there are times due to weather and life circumstances that I have no other options.

Newer insecticides stick better but can afford to lose less potency after application so it pretty much evens out, as I recall, except that Imidan has kick-back.

I think 4 Tb/gallon rate seems very high.

I think the ease in which home growers may accept applying pesticides at 4X the legal rate speaks a lot about why Imidan is restricted in some states. If nothing else, it quadruples your exposure (and the environments) for no better control.

Although he doesn't mention it in the article below, I also recall his research indicated although Imidan does wash off, it has a high toxicity to target pests, so that a small amount of residue will protect fruit.

The article also shows his decision chart. He shows Imidan need not be reapplied after an inch of rain unless the rain occurs 7 days after the application.