How much Benadryl (diphenhydramine) should I give my 4 year old daughter who weigh 29 pounds?

One teaspoon =5ml. 5 ml=12.5 milligram of children Benadryl (diphenhydramine) liquid is appropriate dose.
0.5 to 1.0 mg/kg. So if she is 29lbs she weighs about 12-13 kg. Benadryl (diphenhydramine) liquid comes 12.5 mg/5ml. I would say 5 ml would be a good dose. If she has never had it before you can start on the lower end with 3 ml and see her response.
Benadryl (diphenhydramine) is sedating. As you likely know so it is important todose appropriately. A number of sources are available, here is one: http://www.thhpediatrics.com/wp-content/themes/thhpeds/pdf/DiphenhydramineOption1.pdf.