I suppose the big Channel 4 programme I don't believe S4C ever showed was Channel 4 News.
That's right, but only the main 7pm edition, I think they did show the lunchtime edition when that existed.
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
S4C's viewers were savvy enough to notice the relationship with C4, so wouldn't have been confused by references and its branding, however the reverse wouldn't have been the case, so there was little to no S4C references on C4 shows