This is debatable. I split all my proscar within a blister strip. Thus 10 pills become 40 pills.
Ideally, it may be preferrable to split one pill at a time, and thus preserve the integrity of the medication, until you are ready to use it in the next 4 days.
Most medication should be kept in a cool dry place, preferably in an opaque plastic/glass bottle,(screw capped with a bit of cotton under the cap) in order to reduce thermal decomposition and photo degradation.