I used to read everything published here on the subject of small dishes and C-band reception.
The 4-foot dishes are interesting, in that they can get some signals, but really can't reject interference from adjacent satellites.
So, "do they work?" Yes ... but they are limited.
One of the users here has some comment in his signature about using a 4-foot dish for C-band.
It's better than nothing, but don't get your hopes up.
As for 6-foot dishes, one of the moderators here, Iceberg, has a center-fed 6-foot dish.
From his many reports, it sounds like that's the minimum size for North America.
Bigger would be better, but it gets by most of the time.
Not so good on 7/8 FEC transmissions, or in crowded parts of the sky.
Note that most prime focus LNBs are f/d of .4 or so, and most offset-feed dishes have an f/d of .6 or .7
This is a big problem putting typical .7 f/d Ku LNBs on prime focus dishes.
However, putting a .4 f/d C-band LNB on a .7 f/d offset feed 6' dish may be a lesser problem.
Users of offset-fed dishes generally seem to need conical scalers.
Maybe that is the big band aid. (?)