The relay data sheet says 5V and 208 Ohms. It doesn't say anything about current requirements so I called them and the guy told me it needs 13mA to operate.
Try as I might, I just couldn't figure out the low-output current (i.e. 13mA on pin 4) problem so I decided to use a transistor approach and eliminate the opto completely based on comments from members of this forum. With a transistor I only need a couple of mA out of pin 4 to drive the coil at 20mA.