im prepping a 4th gen rear end to go in my car. will the brake lines from the 3rd gen transfer over to the 4th gen? the line one the 4th gen are damaged.

 

Unlikely

 

You can buy pre bent lines for a fourthgen relatively cheap still. The thirdgen lines will be to short

 

Its the rubber hoses that go to the drums

 

You must be talking about the single rubber line that comes off the body and connects to the T in the hard lines because both my 3rd and 4th gen rears that had drum brakes are hard lined all the way to the drums. So if you are talking about the single rubber line then I used mine from the 3rdgen rear with no problem.
If we are not talking about the same line I would wonder if your rear had disc and was switched to drum or if someone switched out the rear from a disc car maybe

 

Rear drum brakes are hard piped from the brake cylinders to the Tee-fitting at the differential, then there is a flexible hose from the Tee-fitting to the hard line at the chassis.

Sounds like you have an improvised setup.

in-line tube has pre-bent hard lines for many makes and models. (with option for stainless steel)
Make sure you have the OEM tabs still on the axles to hold the new hard lines, if not you can weld new tabs on or use zip ties