For all: 4200 is the WRONG sealant to use for
hatch lenses.
Polyurethane (4200, 5200, etc.) in general have very poor adhesion to plastics like
acrylic and polycarbonate.
Proper glazing silicone is the only right answer here (even for those people who HATE silicone!) Dow Corning 795 or GE SCS2000 are good choices. They stick well to both
aluminum and plastic.
You can wait until the seal on your hatches fails and the lens falls out. Of course if that happens
offshore in a storm you MIGHT decide that preventative
maintenance is the better approach.
Never buy a
boat who's previous owner had the motto "if it ain't broke, don't fix it." You can be sure lots of stuff is ready to break!