I have 2 -3 axis H20T43-SRV400 boards.Is it possible to make the 4 axis board with the electronics I have on hand?
yes and no. you have isa cards which should work, on the card there are 3 large ic chips and one empty socket all you need is one more chip and you can controle one more axis, if you have another amplifier card to power the servo. as well as the aditional motor cables etc.
if you have a chip puller you can remove one chip from one board and add it to the other. watch out for static and bent pins.