I am adding a new kitchen with a sink drain that will run 25' to tie into the old system. The system is vented with a 4" stack through the roof. Do I need to add vent where the new sink begins?

Do I need to add vent where the new sink begins?
Not where the sink begins but five foot downstream from the trap you will lay a sanitary tee on its back and revent back to the 4"stack or run it out the roof.
If local codes allow you might just save yourself the hassle of piping a vent and install a AAV, (Air Admittance Vent) just downstream from the trap in the cabinet.
Good luck, Tom