I tore mine in Dec '01. Since there was no swelling the ER doctor decided it was just a sprain and that I should let it heal for a month or so, and see a doctor later if I had problems. Started riding lightly a month later and it seemed ok, but it would tweak occasionally if I put my leg down straight or with weight on it. Problems got fewer by spring so I let it go, but I tweaked it bad at whistler that summer, and I also noticed tweakage when I tried to start running. Riding seemed to give the least problems. Finally went to see a doctor to get a referral. The earliest booking for an initial appointment was 11 mos. The knee doc, of course, took all of 3 minutes to figure out what was wrong. Incredibly, from there I had my choice of surgery dates beyond two months wait so I picked one in January.
The whole point of this is that your muscles will compensate for the lost ligament, and your knee will eventually stop tweaking so much. It will continue to improve without the ACL the more time since the big crash. Riding is also low impact compared to other sports and as you rarely fully extend the leg, it can stand up to normal riding. By that I mean you will eventually be able to ride just as before. I hauled my torn ACL to all 6 BC cups last summer, Whistler 25+ times a year, and everywhere else all over this year and last, and my riding has continued to progress.
Unfortunately the knee still has no stability in that one direction, and while it may feel fine there is the always risk you will blow out the rest of the knee in a bad crash. You will be able to ride the skinniest skinnies, launch the biggest airs, and go as fast as you want. The biggest problem you will have is hesitating because you are scared of hurting the knee again by crashing. Ironically this will cause you to crash more. Just take it slow, let the knee heal a bit, and be careful. You only have 4 mos to wait. It will be 2 full years for me…. its not that bad. Good luck