If you do plan on migrating to
vB 4 that would be the first step before adding anything new. Once the migration is complete then add in your mods one at a time and test between each.
The
vB license does allow for a test/demo install along with the production version. The problem your plan will cause is the db will be out of sync. The test and the production db's will have different data if both are running at the same time.
The method we use to migrate to
vB 4 is to created a test install by copying and then renaming the production db. The adjusting the config.php to point to the proper db. Install the currently used
vB version, in your case 3.8.5, then update that test site to
vB 4. From there add the mods desired testing after each.
Once comfortable with the test side of things then it's time to shut down the production site, run the update and install the mods.