My GI Doc Prescribed And Gave Instructions For The 2

my GI doc prescribed and gave instructions for the 2 part prep, but ok'd the pharmacy to give the big 4 liter jug(was less expensive). Should I still split it into a 2 part prep ( 1/2 the night before and 1/2 the morning of procedure) or should I drink all 4 liters the night before? Have colonoscopy and upper endoscopy at 1pm