Using 4xAA nimh

[iSP]

However the pi and the rest works at 5v! if you want to calculate the total capacity at 5v do it like this;

(3 * 2500 mAh * 3,6v ) / 5v = 5400 mAh.

I don't think that's right. It should be:
(3 * 2500 mAh * 1.2v ) / 5v = 1800 mAh.

When batteries are in series their mah does not increase.

You're right about this one, already did 3x 1,2 for the 3,6 which I shouldn't, so there was a factor 3 to many.
Still it should hold up.
Mental note; shouldn't do maths near bedtime