I just bought a used 4U AT900P and it's had been strung consistently at 26 X26 lbs by it former owner. It was manufactured in 2008, so it might have been strung at 26x26 lbs for more than a year or two. It's still intact but now I strung it 2 lbs lower. So, I know 26 x 26lbs is O.K. for a 4U AT900P. Anyone with a higher tension?
From the Yonex specs, it's 24lb, though as already indicated you can string it higher but it will invalidate your warranty.
27lbs should be fine, I have been told by friends who have strung their 4U AT900 (both versions depending on the person) consistently at 28lbs and not run into any problems with frame fragility or similar things. Another thing to consider is that some 4U are actually "overweight" meaning that they can weight almost as much as the lightest 3U but are still classified as 4U. Either way it should do fine as long as you don't abuse its frame strength or clash it.
