What if Poland,Lithunia and Prussia had a personal union?
The POD is late renassiance
I actually want a Brandenburg-Prussia-Poland-Lithunia personal union how possible is that?No reason for that, since in late renaissance Prussia was Polish vassal. Going few decades earlier we might see Teutonic Order/Prussia directly annexed instead of vassalised, but even then it will not be separate entity on par with Grand Duchy or the Crown. Prussia wasn`t really important, strong or influential before times of Branderburg-Prussia.
I actually want a Brandenburg-Prussia-Poland-Lithunia personal union how possible is that?
It's actually not that hard, Prussia keeps its part of the Poland partition instead of losing it to Russia, resulting it having a more eastward instead of German focus. At some point it weakens because of nationalist strife, and forms akin to the dual monarchy with Poland on the model of Austria-Hungary.