How much worse could it be than running a 4" block without a shackle flip? Granted the proper springs would be best, but I can't see how the 4" blocks suddendly become so much worse with the shackle flip.
BTW, I've been running 4" rear blocks in my 1-ton, fully locked K5 on 38's for almost 10 years with no issues and I've broken 2 front D60 30-spline stubs, so it doesn't get babied.........though I do have the shackle flip kit sitting in the garage.