What e6bpilot said. Open time reserve is paid as a stand alone block, or value of the trip assigned, whichever is greater.

1, 2, & 3 day reserve blocks carry a block liability of 6 hours/day. A 4 day reserve block liability is 20 hours. You can't pick up more than 98.5 block hours in a 28 day period due to the company buffer.

If you've flown a lot and but have room for 20 more hours (assuming nothing else in open time or trip trade you want) picking up an open time 4 day reserve block carries a decent chance of getting a 3 day trip or less that pays 24tfp. A roundabout way of getting premium credit.