You already have some pressure and just to increase the pressure to 40 PSI and 4 GPM You need 48 watts of energy at 100 % efficiency, for 50 %over all, You will need 100 watts.
Motor burn out can be caused by a locked rotor or too high friction due to debris in the water.
You are not clear with the whole system, one needs to remember that the only person that knows the system is You (or am I wrong here ?.).
Give an accurate detailed description step by step to see what is going on, this way a possible realistic suggestion can be given.
Something is wrong with your statement :
>Pump inlet pressure via gravity flow through 1 inch pipe from cistern is 15 psi and 2 gpm.
Can You accurately tell us if you have the water volume and the pressure at the same time, or is the pressure static at zero water flow and the volume of 2 GPM when the pipe is in full flow fully open ?,
Because 15 PSI is 10.6 meter head, that with 1 inch pipe, the open pipe flow for such short distance would be 7.2 liter/sec or around 115 GPM.
How old is the pipe in the system ?.
Is the water clear with out any sediments that may place or glue "Cholesterol" on the pipe walls ?.
If the pipe is clogged the pumps may be fighting a restricted flow causing over heating and motor burn out.
Nando