I test drove several c3 and c4's before I bought my 73 Satellite in 2012, because- "just so I can say I've owned one".
The c3 is a much sexier car, even from behind the wheel, but- the c4 is a much better car from an engineering standpoint.
Back then the best deal I found was a black 93 lt-1 six speed manual without traction control (which I understood was desirable) for $5,500.
I saw several nice c4's under 5K, and nice c3's were 4K, the nicest being a black '75 at a Dodge dealership of all places, or a 73 vert needing reassembled after paint for $3500! I also saw a manual trans c3 "Pace car" with nice paint and interior for 7K, and an '85 convertible manual trans (4+1) for 5K. I learned I don't care for the 85 MPH "Knight Rider" dash.
I did find the c4 way more difficult to get into and out of vs the c3.
The c3 is just low, but the c4 has a very high sill and the door is more rearward with respect to the seat and footwell.
Fans call it "fall in, climb out" and that's exactly what it feels like.
Once inside, the c4 had slightly more headroom, but my head did hit the roll bar under acceleration.
Prices have ticked up in the last 2 years and a "**** box" c4 now starts about $6500.
There is an 87 auto down the street with paint that is starting to go, and a trashed interior.
It was listed at $6500, but 3 months ago they jacked it up to $7900. This is a buy here lot, though, but it's what I'm seeing CL ads similar to.
The best c3 deal I see today is a 77 with no engine and faded paint but looks solid (say it with me- ls swap) for 3K.