What is the maximum water speed from a compressed watergun?

[PFCJeff]

Homework Statement

A child's water cannon consists of a water chamber that can be compressed by an air pump. If the air above the water in the chamber is compressed to 4 times atmospheric pressure, what is the maximum speed of water from the cannon when the trigger is released?
Ignore any height difference between top of water in the chamber and the opening of the barrel.

Homework Equations

The Attempt at a Solution

All i can figure is 405.2 kPa...but I'm not sure how to apply this without any other variables. Any help would be appreciated.

 

[PFCJeff]

Oh, I might have figured it out...

I used Bernouilli's equation... constant= P_o + rho (v^2+2gy/2)

so...
4P_o = P_o + rho (v^2+2gy/2)

then i isolated for v to get the speed... is this correct?