I´ve converted a few No 4 fore-ends to No 5s, and have found that best results are obtained by free floating the fore-end from the knox form forward. The No 5 fore-end is slimmer and does tend to warp. On the other hand, the barrel is shorter and and is therefore less prone to resonance. Think it must be accepted that a No 4 will always be more precise. You might be able to re-seat the action slightly (and maybe remove a minute amount of the offending wood from the fore-end) to obtain a free-floating barrel.