My recollection is that Chris Evans would often say something like "you're watching the Big Breakfast on Channel 4", coming back from a break for example, so I'm wondering if that irritated either Welsh viewers or S4C management?
As mentioned, S4C were happy to make use of C4 properties. We lived on the border and when we got the game cards to play along with Housey Housey on The Big Breakfast (which surprised me greatly by having Mark and Gaby on the front, rather than Chris and Gaby), the ones that came through our door had S4C branding, rather than C4.
Another thing that was there at the start was the BBC and HTV supplying it with Welsh programmes, taking the Welsh language programmes off their own channels. That still is the case (though I don't know how much ITV supply now) although the relationship with the BBC is different as S4Cs funding comes from them now
ITV Wales still do a few programmes for them, Y Byd A Bedwar, the long-running current affairs show, started on HTV forty years ago and is still going today, produced by ITV Wales.
The only reason I mention it was that there was apparently a great effort to cover up the C4 branding during the ITV Schools roto sequences.
Presumably one reason why they had to be very explicitly S4C branded was it was possible in quite a few places in Wales, like where we were, to pick up both, so it was important for schools to know which ones they were watching so they didn't acidentally record the S4C output when they were trying to record a C4 programme, or vice versa. For a while in the early nineties S4C had a
DOG on all their English language output, but it only lasted a few months before being dropped due to numerous complaints.
S4C had two main functions to perform when it started. It had to take on the BBC's and HTV's Welsh language programming, (the BBC had to provide theirs free of charge) and it had to show, either live or timshifted, as much of C4's programming as possible. Obviously it couldn't show everything, and it would have been nonsense to timeshift C4 News to 11:30pm
One other main purpose was to include Welsh language programming in peaktime. Before that it was the worst of both worlds, because every time the Beeb or ITV showed a Welsh language programme in primetime they had to displace an English one, to the displeasure of those who didn't speak Welsh, but then it meant if Welsh programmes were just in off-peak slots, it was a poor service to Welsh speakers. I've got the Welsh Radio Times from Christmas 1981, the last Christmas before S4C, and on Christmas Day all the Welsh language programmes were in a block on BBC2 from 9-11am, while in the rest of the UK it wasn't broadcasting. That was convenient for English speakers, but not very good for Welsh speakers.
So the idea was that you now had three channels entirely in English, while still having Welsh language programmes in a prominent spot. Of course, that just meant the arguments about having to stay up late to watch English language stuff on BBC1 and ITV was just replaced by arguments about having to stay up late to watch English language stuff from C4, but at least that wasn't previously available at all in Wales.
There were occasions when S4C made a bit more about C4 shows, on SOTCAA they mention how they started promoting the comedy show Absolutely a bit more, making their own trailers for it, when they realised it featured the Welsh performer John Sparkes and there were often sketches set in Wales. Some of the Absolutely sketches did, as a joke, have Welsh subtitles in the programme, and S4C promptly did their own, correcting the spellings, for the trailer!
And vaguely back on track, was "Open College" the ITV/C4 version of Open University? Did it air just in weekend slots?
It was on C4 at lunchtime and also at TVam at weekends, although TVam transmissions were paused for quite a while during the strike, which didn't really help establish it. Open College, as its name suggests, specialised in more vocational education than the OU.