I have a bathroom on the second floor and need to move the toilet 4' from existing roof vent and drain. Can I use the existing vent.

Sure, you can. The toilet pipe is either 3" or 4" in diameter. For 3" pipe, you can be up to 6' away from the vent. For 4" pipe, you can be up to 10' away from the vent.

If you are moving it only 4' away from existing location than you should be fine with venting.

sure, you can. The toilet pipe is either 3" or 4" in diameter. If you are moving it only 4' away from existing location than you should be fine with venting.

Thanks I need more info once I see what's under the floor.

Sure, open it up, including walls, take few photos and post it. We will lead you through the installation...