You have a 4.0 with a weak schedule. The adcom will also see 4.0 candidates who took 17 credits every semester and applicants who took 18 credits every semester and earned a 3.96. Given that medical school is a heavy schedule that is usually non-negotiable (you can't take a lighter load, it is often all or none), do you think that the 3.96 with 18 credits will be considered a stronger applicant than yourself?
Is there a 3 credit course you can take as filler? Music appreciation? art history for non-majors? Ask around.