Anyways...
The Gopro Session, although it is not indicated as 4 but as "session", is the replacement of the previous Session 4.
The video quality in terms of range optic and sensitivity is definitively not as good as a Gopro3+ Silver, which I used previously.
Range : lots of bloated/saturated areas in the Session. That also may be because the contrast is excessive, anyways my Gopro3+ silver was better with less oversaturated areas.
Field of view : My Gopro3+ Silver had a more linear, less distorted field of view in Medium. The 'medium' equivalent in the Session shows curvature and has a shorter focal length than a 3+ and as a result, objects shot in 'Medium' in the session are smaller than in a 3+ Medium.
And I also think the gopro3+ was more sensitive in low light/green light than the Session.
Now I'd expect a comparison with a Gopro4 classic of any type to be worse for the session.
Plus the limitation at 30 feet is too restrictive and without a waterproof housing case, expect lots of scratches. The bottom line : you need a housing.
Plus, even if you find a case for your Session, which I did, the compatible filters will have to fit that cubic housing shape which results in bigger filters than those for classic gopros. Altogether, a Session + case + filter, is a bulkier system than a classic Gopro + housing + Filter.
In short, I don't know how a Session 5 would compare, but the session(4) is not worth the money IMO if you already have a classic gopro.
The only advantage of the session is its price $149. It works, OK, it really is a shoot and forget cam that didn't gave me any problem. Battery life was good 90 to 120 minutes, in 50 degre water. I had no problem really except the software issue. I have taken a video to prove it :
https://youtu.be/uX3bR0iE4mk
Taken at 1080 X 30fps/ Medium + light Magenta filter.
Not sure the chip is sensitive enough to use it at 60fps with a filter at 15ft deep in green water.
How nice you may (or not) think of that video, believe me I definitively feel it's a downgrade from the classic 3+ Silver.
So you know.