Got the cover late yesterday, and fitted today.
It's very lightweight (so it packs down small - in its own little zippered parcel) so I think it's designed to be keep showers at bay rather than a serious storm (have to put the soft-top back on for that).
I can't say that it fits a TR4 very well - it's ok, and covers the top etc., but it's obviously tailored for a different shape of car. I haven't yet tried it on the car with no top (when it would be held up on the screen and the rollover bar), but I think it would look a bit more ill-fitting but still probably cover everything ok.
Part of the problem are the elastic straps that hold it on - they don't naturally fit a TR very well, and I've hooked them under my wire wheel nut "ears".
I suspect it will do the job I want (keep any light rain out when parked so that I don't have to raise the top) but there's an opportunity for something MUCH better for our cars...