Am I right in assuming that 2.xBSD was the state of the play on PDP while 4.xBSD was the source tree compatible state of play on the VAXes?
Sort of/in theory and all that.
2.XBSD was initially developed by a group of folks that had already invested in PDP-11s (in mainly separate I/D based 11s) and could not afford a new Vax. The 11's are address space-constrained and with the introduction of the Vax, one of the side effects of BSD was much of the Unix 'small is beautiful' / 'do one job well' / 'KISS' ideas started to be lost, i.e. Rob's super 'cat -v harmful' became necessary to write (along sadly was often ignored).
Since CSRG (EE and CS at EE) abandoned the 11's pretty fast, there was a group (originally lead by Keith Bostic in the Stat Dept) that wanted some of the new code (particularly the networking stack and sendmail) moved to their 11's. In some ways, I was surprised that it has kept going, as the 68000 & later 386 based UNIX systems came to the world, as the economics of running an 11 started to dwindle quickly.
That if you had a VAX you got the 4.xBSD tapes, whereas if you had a PDP you got the 2.xBSD tapes?
If you were a University or Research type that qualified for a $100 style research license, you would get a pure V7 (PDP-11) or a 32/V(Vax) tape from AT&T patent and license. Once your site had that, you were part of the source 'club' and could whatever you wanted based on the AT&T V7 license. So if you were interested in the BSD releases your team then contacted the 'ILO' (UCB's Industrial Laison Office - BTW CSRG's worked with the ILO for all the BSD tapes) and asked to obtain UCB IP (be it the CAD tools such as SPICE, or the OS work like BSD, IC process technology, et al). There probably was some sort tape writing, i.e. short fees, associated with the specific IP on the order of $100-$1000 depending on what you requested, and there might be some licensing steps (exchange of AT&T license signature pages). When you had a license from the ILO, you were part of the UCB 'club.'
The original BSD and 2BSD tapes themselves were released officially by the ILO, as with 3BSD and 4/4.1BSD. By the time of 4.1a BSD and later, we had CSRG, and those releases were done by them directly after the licensing was set up by the ILO. As part of the funding and creation of CSRG, UC Berekely finally had a C30 IMP in Evans (as opposed to the VDH to LBL), so the releases were also possible via ftp from a hidden location on ucbvax. 4X originally targetted Vaxen, but famously other systems like the 386 we available on that site.
By the time of the 2X releases, UC Berekely had the C30 IMP ( i.e. direct internet connection). So, once you were licensed, you got the keys to be able to FTP different 'tapes' (which included sources and binaries), be it 2X or 4X base But, since CSRG stopped focusing on 16-bit, the 2X stuff became more of labor of love and was a bit less formal and was done with cooperation with the CSRG team.
So ... if you owned a PDP-11 and were still running it and you had a proper UCB license, then yes, you might be tempted to run 2X; but the truth is most people began to turn them off in deference to more cost-effective platforms.