4oD ads are definitely poor quality. The video quality of the actual programme is a different story however.
The best I have tested was on the Samsung Smart TV, which streamed at around 2400kbps. This is pretty good, and in my opinion the video was better than SD, but not quite HD.
On the Now TV box (and I presume Roku), I tested 1200kbps, so about half as good and slightly under SD.
Which platform are you using?
As for the amount of commercials, to be honest, they still play a lot less than what you will expect when watching on a real TV broadcast.
And those commercials are how channel 4 makes their money. Remember, they don't get money from the license fee and are entirely funded by advertisements. Take the ads away, and you kill their revenue.
4oD offers quite a lot for nothing. It even has box sets of entire television series, and that is pretty impressive. I can't think of any other free catch-up service in the world that also offers that, especially since they are major shows that normally sit behind a pay-wall (Netflix, Hulu, Amazon Prime etc). That's what those ads pay for.