can i put front line flea treatment on my 4 week pregnant bitch?

Personally, I would never put flea treatment on a pregnant bitch, or one that is feeding pups. If she has fleas speak to your vet. I usually buy a nit comb from the chemist and brush it through the dogs coat. Any fleas found are then drowned in water, but you have to be quick or they will jump off

No - don't put frontline on your bitch whilst pregnant &  when lactating.  :)