Frequency control and droop
In this case, if we neglect the frequency dependency of the demand, all generators are going to be loaded with their rated power as the deficit in power (70) is equal to the left margin of G2, G3 and G4. However, to get more accurate results, we should take that dependency into account.

Mathematically it can be solved as follows, as soon as you get G3 power (156.51 MW) you can set its power to nominal power (150 MW) and redistribute the (6.51 MW) between the G2 and G4. This can be done by following the same procedure of losing the 70 MW. I got the following numbers PG2=199.753 MW and PG4=297.037 MW.