The answer is yes, but why would you want to? You can get a 4 speed crash box, that would give you a granny low and no syncronized gears. The 3 speed is much more driveable.
If you are considering a more modern 4 speed, it will take a little more work. The bolt patern in the front that mounts to the bellhousing is different, and there is also the problem with the torque tube rear end. For the hassel involved, you would probably be happier with the 3 speed, or converting it with a T-5 5 speed. That is just my $.02.
