There is quite a bit of content in HALion 4 that is NOT a part of any Cubase or Dorico distro that I am aware of (more than 700mb worth of programs and samples for H4 alone, plus whatever advanced Sonic stuff [close to 10gig worth?] that came bundled with H4).
Note the two expanded Libraries in this image…
In my opinion, the H4 content is quite good and useful, as I still use quite a bit of it on into the age of HALion 6! Much of that H4 content is unlocked in a way that you can edit and tweak it to make your own sounds. It’ll also give you abilities to import your own samples and such (perhaps even from things like Akai and Roland sample disks), which you cannot do in any version of Sonic.
I didn’t get into HALion until version 5, but…assuming HALion 4 came with a 64bit plugin wrappers, I personally would try installing it. The only reason I can think of not to give it a try would be if you are extremely short on disk space for the additional H4/Sonic content.
My ‘rough guess’ is that adding H4 to your setup would probably require roughly 10gig or so for the additional HALion/Sonic Library content.
Cubase 10 will not accept old 32bit plugins without some sort of 3rd party bridging process, but I’m pretty sure Version 4.5 came with a VST2/3 wrapper that will work with 64bit DAWS?
If you should decide to upgrade to HALion 5 and/or 6 at a later time, your HALion 4 plugin will go away and be replaced by the newer one, but all your content, user programs, and DAW projects should keep on working in Later versions of HALion just the same.