If the gear ratios are different between the 52 and 54, then the syncros may be different... Though 4th gear should still be a 1:1 ratio I can't say if the syncro would be the same....
I'm pretty sure that a G54 is just a G52 with heavy duty bearings, the syncros should be the same, and Marlin Crawler should be able to tell you if that's right. The G52 came behind 22R carbed engines from 84ish to 87ish