I know this sounds academic but if you don't do what's right, you may end up doing it again later.
The problem with using broken up concrete as your base (in this particular case) is that over time the sand will break out of the pieces and may clog up the weep holes in your drain pipe. Plus, it will settle over time and eventually you'll see the ground sinking around the area of the drain.
The proper gravel is 3/4" crushed stone. Any gravel pit in your area has it. (See the Yellow Pages under Quarry or Gravel.) In my area, 1 ton costs $75 delivered. $35 per ton and $40 for delivery. So if you know of a future project where you'll need gravel, size it up and have it delivered all at once.
Now, as for the drain being 4' deep, there is no exact depth unless you're draining along a foundation, or something like that. Since you didn't say what the french drain is for, I'm gonna assume you're just taking excess water (from downspouts or whatever) away from the house, and possibly off the lot. As long as the pipe has a slope to it, 1' or more is OK. The deeper the drain, the more forgiving the slope can be because of frost and freezing (if this is an issue where you live.) The close to the surface, the LESS forgiving the slope can be. At 4', you'll need a minimum of 1/4" per foot of slope. There's more ...
I strongly recommend that you line the drain, first, with filter fabric; NOT landscape fabric. (Ask the Quarry folks. They should have it or know where to get it. Make sure it's NOT landscape fabric.) Then, lay in 3-4" of gravel. Then, lay in the pipe with the holes facing DOWN. Then, wrap the excess fabric over top of the pipe. Essentially, you've wrapped the gravel and the pipe in the fabric. Then backfill with dirt. If you're also building a pit to collect water where it's being taken away by this drain, line the ENTIRE pit with fabric. Go ahead and be generous! Leave the top 6" of the pit unfilled and lay over more filter fabric. Then backfill the rest of the pit with dirt and grass.
Another idea for the pit is to, again, line it with fabric and fill it all the way to the top except for the last 2". Lay over more filter fabric, and then lay 2" of landscape gravel over top. The fabric keeps the stone (and any piping) from getting clogged with dirt, leavers, stones, etc., etc.. SHould the top 2" of gravel start looking yuchy, remove it, then clean off the fabric, and put the 2" of gravel back down. Here's a link to give you a picture. How To Build A French Drain. My best to ya and hope this helps.