I think this was the case a long time before the 5.4 beta, but final gather causes a “your application is out of memory” error, even when set to 128 rays. I have 32gb of ram, but even a moderately large scene cannot complete lightmapping with final gather switched on. Without final gather, the results are unusable.
Do you have a repro project where we can assess? Or a bug report with more details?
Please post a bug report # so we can investigate further.