1 liter is 1000 milliliters. To make a comparison or do math, you need to be looking at the same units of measurement. 
Ben bought:
4000 mL of water
3750 mL sports drink 
3350 mL of juice 
1.) 4000 mL water - 3750 mL sports drink = 250 mL more water 
TRUE
2.) 4 L water - 3.35 L juice = 0.65 L more water 
FALSE 
3.) 3750 mL sports drink - 3350 mL juice = 400 mL more sports drink
TRUE