Thanks, didn't realize there were different, and doing some reading online, apparently incompatible options.
You get what you pay for ...
The Assembly tools are more compatible than you may think, but folk are correct in that you should not mix workbenches, even though they are capable of running independently alongside each other. Also, in this case, you get what you feel like having, and you don't pay a cent!
From what I have seen, Assembly 2 workbench was becoming very capable, but there were still lots of guys adding their own extra tools (something you can do with FreeCAD Python scripts). The developers incorporated a whole bunch of "wouldn't it be nice" features, and built
Assembly 4, which did make some Assembly 2 features disappear, being rendered unnecessary by the nature of the new concepts.
I do not know exactly at which stage the new Assembly Workbench came in. The "stable" version of FreeCAD is 0.19.3 I use "FreeCAD Daily 0.20. FreeCAD is available for Windows Linux and Mac.
I use Linux, so the "version" number you see is from a Linux repository. To get FreeCAD Daily, I needed to add the freecad-maintainers ppa. This would be the so-called "untrusted" Personal Package Archive, but that is only because Ubuntu and other distros have repositories distributing at versions somewhat behind what the developers have already proven. I have not seen a problem with the developer version, and it has incorporated lots of fixes to remove annoyances, and add nicities.
If you happen to use Linux, then the way to have your software choose FreeCAD-Daily when you do an update and upgrade is like so..
sudo add-apt-repository ppa:freecad-maintainers/freecad-daily
sudo apt-get update
sudo apt-get upgrade
The update command will refresh the available update status of all your software, including FreeCAD, and the last command will actually go ahead and update it. If you are going this way, I would uninstall the old FreeCAD first. You don't have to, but it helps in getting the right one launched without confusions.
Unlike Windows, the whole scene is strictly a download only, (no "Telemetry"), and it happens alongside what you are doing (no forced re-boot), and is over in a minute or two.
Look at the Add-Ons Manager
Regardless of all that, check out Tools--> Addon manager. (or wherever you find the Add-on thing)
See what versions of Assembly Workbench there are. If
Assembly 4 is there, then use it. If your model was done in Assembly 2, then stay with it for that model. Clearly you can have both workbenches present, (but use only one at a time). I have no idea what might happen if you go at modifying an Assembly 2 model using Assembly 4. I have not explored Assembly 4 yet, but there are YouTube videos on it.
Updating Older saved Designs
The pictures are a portion of what I see when it comes to available free feature tools. They install automatically in a few seconds. If you select them, they pop up a warning notice if they have been overtaken by one of the other later workbenches. I think the reason they are kept available is so you can get at, and modify, a model you may have made in the past. At least once, I have encountered a Workbench that gave me the instructions on how to "change" the saved model, to default to the later tool.
_
As you can see, right at the top, I was getting interested in the
3D-Printing Workbench, and
Assembly4 is selected, and I could not resist the "
Bolts" and "
Fasteners", and especially the "
fcgear" automatic gears generation tool.
While we are showing pictures of our FreeCAD models. here is an older picture of my South Bend 9C countershaft and drive wheel. It's not obvious, but the taper pin is also exactly modeled.