I've got a few of those. The 17 and later model 4-20 (4 hp. 20#) were the same, and there was also a model 5-20 that used a lot of the same parts but had a bigger motor. Most of the 17's used a Tillotsen carb, but a few used a Brown, most of the Brown carb versions were junked early because they had an awful lot of problems.
Like many saws of the period, used a gear drive and a mechanical air-vane governor to limit RPM. Engine is about 4.3 cubes, revs are limited to about 5500 by governor. Bars were available from 15" to 63", but most common in my region was 18"
Compared to modern saws, yeah, they are big and heavy. I fired one of my old clunkers up a couple of weeks ago and they are LOUD. Because they are gear drive, you can't hardly slow them down, even though the chain moves pretty slow to begin with. I was able to slice off a 15" half-dry red oak in about 24 seconds by jamming the huge felling dogs into the log and forcing it to cut.
Just as a comparison, my newer Homelite super-EZ made a cut in the same log in about 18 seconds, while my Jonsered 2149 did the same in 11. To be fair, the S-EZ needs a better bar...the original Homie bar's groove is a bit wide; it's cutting toboggans.
I embarrassed them all with the husky 395!