Do you do shorten the very long scheduled rides when on Turbo?

I am trying for my first Ironman in June (Austria). I am at the point now where my long rides are getting pretty long at nearly 4 hours.

I've done a 3 hour ride on the turbo trainer and it was OK but was a bit of a grind.

I've read that some people say it's acceptable to shorten the rides sometimes, as the turbo is continual pedalling, with no rest, no coasting or downhills.

Just wondering if this is something you'd agree with?

I'm in the UK and it's been raining for about two weeks now. It looks likely that Saturday will be raining and I'll do the ride inside again. Just trying to plan ahead.

Thanks