Is there a transistor that acts like a relay?

[JerryG]

I need some kind of four pin transistor that is neither common drain nor common source. Both positives and grounds need to isolated. Does that exist?

[Studiot]

Try one of these

http://en.wikipedia.org/wiki/Silicon-controlled_rectifier

You could, of course explain your application further for more detailed advice.

[Averagesupernova]

They are called solid state relays. Usually an optocoupler in conjunction with a triac.