Where can I watch the turtles lay their eggs in Costa Rica and Panama in the second half of April? I am mainly interested in places that are accessible by car (4x4 is fine).
There are at least 2 tours that I could find that could fit your criteria:
I am sure that you could probably get to those beaches on your own if you choose to but I think these would be better. There is actually a whole slew of tours just like those at SeeTurtles.org
We ended up watching the leatherback turtles on the Gandoca beach. This was organized through an agency in Puero Viejo de Talamanca (I can't remember the name of the agency). I can certainly recommend this option, and after reading more about turtle watching I think it is actually get by far the highest probability of success in April.