Selling price of the a t-shirt (SP) = Rs 648
Assume cost price = Rs. CP
Losses percent = 4 % = (CP-SP)x100/CP
4 = 100 - 64800/CP
CP = 64800/96
CP = Rs 675.
Selling price for to gain 4 percent => 4 = (SP-CP)100/CP
4 = 100SP/CP - 100
SP = 104xCP/100 = 675x104/100 = Rs 702
Hence he should sell a t-shirt for Rs 702 for 4 percent gain.