Well, I would keep it pretty simple for 3rd and 4th graders. You're right that talking about the venomous snakes might be too much. Good idea though and you can be prepared to talk about it a little if a child asks the question.

Some things I would talk about are where these reptiles are found in the wild (do you have a map you can put up to show them?) and what they eat in the wild and in captivity and how that might be different and how their metabolism is different from ours allowing them to only eat once a week. Plus some interesting facts like how a bp's eyes "blue" before shedding, how they constrict their prey, etc.

3rd and 4th graders are pretty easy to entertain - you should have fun!