Normally the guy who wants the boat in the water pays for the launch--and haul again if they're putting it back on the hard. If they're moving it to the water, that opens up some bargaining.
Four years may have had no attention, so there may be leakage, standing water, bug or rat attacks. Engine might not have been pickled, so the rings may be seized and there may be corrosion problems. Or not.
And the fuel probably is problematic, if it runs it may still be full of glop that clogs the filters shortly.
Surveyor, engine mechanic (most surveyors don't really do engines), verify no liens with the yard, verify the title IS available in hand for transfer from the attorney. And if he knows nothing about boats...it might pay to explain that you expect to pay a fair price, but when boats have been sitting idle the problems can be very hard to see--and expensive to cure. So you need to take a careful look.
He may be more familiar with the concept of setting aside escrow, and that's ont unusual for boat sales. Purchase conditional on sea trials, survey, financing, and sometimes some money set aside in escrow, to be kept or paid depending on whether any hidden problems surface within xx days of the sale.