I’m a long time Poser user (mid 90’s) and have a large content legacy library. I upgraded to Poser 11 in 2015 and didn’t use it much since then. Now I’m interested using it again. Is there any advantages to upgrade to v12 if I’m only going to use my old characters (V4,M4, and before .. Maddie etc) ?

Nope.   The legacy content is the same stuff that has come with Poser forever.  There are, however, other fun things you can do with Poser 12 that aren't available in previous versions.  https://www.posersoftware.com/  and you can try Poser 12 for 30 Days free!

The only less-old stuff added to the legacy content is the HiveWire3D figures. Dusk, Dawn and Baby Luna. As Donnen says. Not worth the upgrade for content alone. There are some P12 features that would make it worth the price. For me the support of more modern GPUs is a big deal. P12 Supports RTX 30 series. And has support for Optix rendering. That gives a massive render speed boost. It also adds the OIDN denoise which can further reduce rendering times. To me those time saving features are a big deal.

If you have a new Nvidia GPU, especially an RTX 20 or 30 series it's worth it just for the 10x faster renders.

P.S. Almost forgot adaptive sampling. Allows you to set high render settings, basically over rendering, but it will stop rendering when it reaches the quality you set for it. Another time saver.

I'm using Sasha-16, in its 2019 version, to render all my Vic4's.
And I'm using Poser12 as:

* it's really faster, especially using Superfly
* Cycles is V2 with many bugs corrected, and new nodes such as PrincipledBSDF
* Poser itself runs more smoothly now IMO

But there's a catch: Poser 11 and previous versions are based on a version a Python that is definitely dead: 2.7, whereas Poser12 had to be upgraded to Python 3.7, Bondware didn't had the choice, while SmithMicro chose to leave their customers in a "nowhere-land", not giving a sh...t...
So: some Python scripts have been updated by cool vendors, whereas other vendors gave up and switched to daz.
Sometime, the .py files may run, sometimes that need to be updated (there are tools for this)
Compiled Python scripts are dead.
If you're using EZSkin3 and others scripts from Snarly, there are updates for Poser12.

So, honestly: it depends on the way you render.

Things not yet mentioned:

* Some small but very useful User Interface improvements (real improvements, not just cosmetic tinkering).

* The Figure Hierarchy Window is now much easier to use.

* A small improvement to real-time Comic Book. Fewer tiny gaps in line-art, as far as I can see.

* The old automatic Poser to Anime Studio / Manga Studio script may not work. Important if you're taking figures to Clip Studio.

* Walk Designer is apparently improved. Haven't tried it.

* The Materials Room displays textures faster. May be useful if you do a lot of re-texturing.

* Support for the latest FBX export. Possibly also supports import of the latest too, at a guess?

* Complete P12 manual, done and finished. Nice.

* No easy DSON import of DAZ Genesis 1 & 2. Poser 11 still needed for DSON import.

* Still runs back to Windows 7. 

* Still exports to Vue.

* Python 3 expands the range of what Python can do. The scripts niggle is now mostly settled for most users, now there are replacements or fixes. For a comprehensive listing of free Python 3 and still-compatible Python 2 scripts for Poser 12, see here: https://jurn.link/dazposer/index.php/p12/