24 vac/dc relay question

No, you use the pins with the correct volt/amps.

In theory, no.

In practice, you'll have to compare the pin layouts to make sure you're connecting inputs to inputs and outputs to the correct set of outputs (that is, do you need NO or NC?). You can't just assume the pins are in the same relative positions with respect to each other on the two relays.

You probably need to give us more info on the application you're attempting...

The 4-pin relay is likely an SPST, and the 8-pin a DPDT. DPDT's can be used as a SPST, but you need to know which pins to use and which to leave unused...

-Tim

Agreed, you also need to know the function of the switch on the existing relay, which could be normally open, or normally closed. As Tim said, the DPDT has everything you need... and more, you need to know which terminals to use for your particular application

OP, isn't there a pin layout on the side of each relay. Or, if you are very careful, and it has transparent sides, you may be able to look through and see which pins go where, but beware, the wires can make a turn you don't see and connect to someplace you didn't notice.

Also when you determine which two pins control the coil, you can connect 24 volts across those two and watch the armature move, or if the sides are opaque, feel or hear it click.