When the tubes are connected, pressures become the same.
Also, the total number of moles before and after the connection are the same.
n1=4V300R,n2=16V600R, total moles = n1+n2=V25R
When the tube is connected:
P1=P2;n1×300RV=n2×600R2V i.e. n1=n2=V50R
So, P = V50R×300RV=6 atm