Yes, it's similar. Usually a solid-state relay is an optocoupler and some other stuff packaged together to make it more convenient to use.
Resistance will decrease, but not all the way to zero.
Actually what will happen is that it will allow a certain amount of current to flow between 5 and 4. As long as you're under the limit, it'll act like a low resistance (about 20 ohms?). The limit depends on how much current you are feeding into 1->2. Figures 2 and 3 in the datasheet give all the gory details. For example, if you apply 5 mA through 1->2 ( If=5mA ) then it'll allow about 60 mA though pins 5 and 4 before it starts limiting the output current.
This makes sense. As long as the stuff you're switching doesn't require more than 50-100 mA of current, then you can treat the optocoupler as a switch. If you need more current than that, you'll need to add something else, like a transistor.
Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1