I have always been told to start with 1/2 mg. per pound, but it was "safe" to go up to 1 mg. as they started to build up a resistance to it by taking it all the time as in allergy control.
So at 4 pounds the dosage would be about 2 mg. which would be about 1/6 of a teaspoon or about 3/4 of a ml so you giving 1/5 of a teaspoon (1 ml) was in the ballpark so to speak.
Here's a link you can read about giving Benadryl to dogs and tells about some of the adverse reactions that can occur. I think it does make most sleepy. Like humans though some may have reactions to medication that hundreds of others don't.
http://lowchensaustralia.com/health/benadryl.htm