I got hold of some used jumbo Jim tyres for my sons on one trails...the original tyres are 4.0 JJ.....the 'new'ones are 4.8"....with a bit of trimming I can get these to fit the rear
What's the worst that could happen if i fit them?
Fitting with the bike static is different to it fitting rolling, ime.
I had to take the 4.8 JJ off the rear of my Dune eventually as it rubbed the chainstay depending on pressure, depending on mud, depending on how much I thraped it down trails.
You might get away with it. Might not.
I've switched back to a 4 on the rear and have a 4.8 on the front.
Yeah, fatbike tyres change shape a lot- when I put a Bud in the back of my Dune, it literally jammed solid in the frame at 15psi (tubeless-fitting pressure) but shrank so much at 6psi that it fitted with space to spare.
The 4.8 JJ is a much better tyre than the 4.0 imo, I'd never go back but in the end a tyre that doesn't fit isn't very good! I don't think I'd cut one to fit, if it doesn't fit without cutting then I doubt you'll get enough clearance that way.
How much clearance is there between the chain and tyre when in the innermost sprocket(s)?
You could go for a 4.4 on the rear