Can I connect 2 water closets and 2 hand sinks to a 3" PVC sewer line ?

Depends on the state you live in really...

It may also depend on whether these fixtures are above ground or if the plumbing for these fixtures will be below a slab.

If the pipes are above ground then all states that I know of will allow two toilets and two sinks on a 3" drain line. If you are below slab, you may need to bump the drain line up to 4"... ;)

Check with your local plumbing department for best answer.

Mark

Mark is correct. It depends upon your local plumbing code. For instance, in Manatee County you can go 3 inch but in Sarasota County it increases to 4 inch. Check with your local building department. Good luck, Tom