Parity bit checks using General Hamming Algorithm
It is done inclusively ie by undertaking the parity bit under consideration, they are designed in such a way that along with the parity bit itself the data word or message bit generates the desired parity type in your case even parity.

Look at the image from wikipedia


Only parity 1 is included while calculating at position one and so on.


  Position 1 Checks data bits -> XOR(1 3 5 7 9 10 12)Position 2 Checks data bits -> XOR(2 3 6 7 10 11)Position 4 Checks data bits -> XOR(4 5 6 7 12..)Position 8 Checks data bits -> XOR(8 9 10 11...)


Remember don't confuse between position and parity number, the bit-stream are numbered (1-n), where n is the size of codeword.

Hence your P1 is position 1,your P2 is position 2, P3 is position 4, and P4 is position 8.