I have , in my posession, a crossbow. I was going to zero in the scope. Is it possible to shoot a crossbow off a bench rest to zero it in. It has a scope with 4 cross hairs. I have never shot one !

Yelp just like a Rifle.Shoot,don't move it adjust the Crosshairs to where it hit.The Top Crosshairs I would set at 20 yards,then try backing off 10n Yards at next Crosshairs down,shouldn't have to move any once the 20 Yard Crosshairs are set.

big rockpile

That's about right except a couple of items. Depending what brand/model scope it is; it may have a speed ring. If it does you need to know how fast your bow is shooting an arrow, to set the speed ring
Also sometimes it is better to sight in at your normal expected shooting distances for hnting. Being as you have 4 crosshair lines you may want to think about the top crosshair sighted at 15yds then 10yd increments for the lower 3 crosshairs

I'm more in favor of 20,30, 40 and finally 50 It's easier to remember rather than 15, 25, 35, 45 and 55. But no one ever accused me of being smart, just a smart a$$

Here's my $.02. The advice given above is good. I'd add that it is wise to check the "zero" of each of the "4" cross hairs. Typically these are indeed graduated off in 10 yard increments, but only approximately. Most crossbows are so fast that the top cross hair is going to be extremely close to "dead-on" from near point blank range to about 20 yards. But it is wise to verify.

For example my Tornado and scope combination is set-up absolutely dead-on at 20 yards. And the Nikon BDC scope specs are supposed to be "pre-calibrated" for 20-30-40-50-60 yards. Truthfully it is a tad high at 10 and 30. About 2" high at 40. About 1" low at 50. And about 2" low at 60. Certainly close enough for a clean kill on deer sized game easily out to as far as I am likely to take a shot.

Thanks guys, It worked perfect off a bench rest. Its a Parker tornado f4. The crosshairs are not individually adjustable. With the top set at 20yds, at 30 its about 1-2 inches high- at 40yds its about 3-4 inches high.

That is off a bit more than I'd expect.

Did you level the crosshairs prior to beginning sighting in?

Just like with a rifle, it's important to make sure you've got the crosshairs leveled for best accuracy.

Just place a level across the rail & also a small one on the top turret cap. If both read level you're good, if not adjust the scope a bit in the rings to get it right on, easy to do.