I (1)/ have been studying (2) / since four hours. (3) / No error (4)

Answer

3

Explanation

for will replace since because
for (Prep.) is used as-for+ Period (duration/of/time) since (Prep.) is used as - since + point (precise momentin time)
for meansfrom the beginning ofthe period to the end ofthe period as in - for 20 minutes since means from a point in the past until now as in- since morning Hence, for four hours is the right usage.