you did most of the problem correctly except for the current that flows through R4. I suspect it is just a writing typo on your part as the current through R3 was correct and R3 and R4 are in series. These are the typical types of mistakes that are made - miswriting a number at one step will yield an incorrect answer at the end. There are numerous methods for checking your own work, though.
One method would be that the total power from the 150V supply be equal to the total power dissipation in the resistors themselves. So the total power of 92.6W should equal the sum of the power from the resistors - which it does not because the power dissipation from R4 is too small because the current is not correct.