vesicles
Colonel
The US also used 4th until they heard the Russians were using 5th, then their egotistical inner lizard brain decided "screw that, we can't lose by naming conventions" so they changed to 5th as well.
China, on the other hand, doesn't give a damn, and is sticking to her own classifications.
I thought the US designation has always been one gen ahead of the Russians and the Chinese (they adopted the Russian designation). So the West has been using 4th gen for their F-15's, F-16's and F-18's for decades before any 5th gen came out... The Russians might have switched their designation to the Western one though...