Back from R&R to a plumbing problem...

I have a problem...started out as a minor electrical problem(buried door bell wire) to a bathroom venting situation.My son is up in the attic looking to see where the bell wire is fed from the transformer.Looking around he see's a loose 4" abs pipe sitting on a piece of 2x6 and the pipe is penetrating the roof.Sure enough,it's right where the vent for the bathrooms should be.(I'm getting waaay too old to be climbing around trusses up in attics!!)Apparently
they penetrated the roof but neglected to bring it up thru the 2x8 plumbing wall.(no hole cut in top plate.)
I took over 2 houses that another contractor abandoned after they were drywalled, on a T&M basis so I'm not getting hurt .(owner dealing with this mess for over 2 years).
I'll be cutting inspection holes wed.
My question:worse case it's broken off at the slab,can I go smaller than a 4" line for both bathrooms(tubs,toilets,sinks share same wall) or do I break out the jackhammer?The original plumber long gone!
BTW,HOW THEY GOT THE INSPECTOR TO SIGN OFF I HAVEN'T A CLUE.
NOT THE FIRST QUESTIONABLE THING I'VE DEALT WITH ON THESE HOUSES...

vent

Unless they were completely incompetent, the pipe should at least be 42" high, and maybe they ran it up against the bottom of the 2x6 to visually simulate a proper vent. You are worrying about something that might not even be the case. But you can take consolation in the fact that worrying really does work, because when you worry about something it usually doesn't happen.

Hack "licensed" plumber...

HJ, thanks for the good thoughts...
Cut inspection holes in drywall after pulling family bath toilet.Not happy!!
Vent pipe about 18" off the slab.About 45" up horizontal vent lines coming in for sinks,showers.Connectors and couplings laying in bottom of stud bay.
After my "good" plumber has a good laugh and the expected WTF,it's done right and drywall reinstalled,taped,mudded.
What's really sad is the original plumber is a licensed guy.
What pisses me off is this house is down to final touchup and I have to fix this mess!!
Wish I had my digital camera with me...