How about the temperature ?
If the temperature is kept as constant then for ideal gas
P1V1 = P2V2
21*16 = 4.1*V2
So V2 can be found
V2 = 21*16 / 4.1 L
If temperature is not constant then P1v1/T1 = P2V2/T2 , We need the values to T1 and T2 to find V2