How to connect old 18650 different capacity cells as a 2S2P pack?
Putting cells with different capacity in serial is not a good idea.
The cell with the lower capacity will run out of energy much sooner than the other.

Because of different resistance the pack could explode if you build your pack as 2s2p.
Imagine 1 serial string with the higher capacity as a charger for the other string with lower capacity. The cell with the higher capacity and thus lower resistance would get overcharged.
If you use protected cells (with protection circuit) the pack or one serial string would shut down when over/undercharged or too much current would be drawn.
But even using protected cells with different capacitance in a serial setup is not a good idea.
Hope this answer helps and you can understand it. My English is not the best ;)

edit: Remember to use a balancer when charging Li-Ion batteries in serial setup.