If I understand correctly, you want to display a checkbox on the create guest page. If checked, unlimited sessions are allowed; if not, the default is used (4 in your case)?
If so, try this:
- Set 4 as the session default (Configuration --> Guest Manager)
- Add simultaneous_use to the Create User form you are using
- Edit the field (not base)
- Change User Interface to Checkbox
- Change Label (maybe something like "Unlimited Sessions?")
- Change Description (maybe something like "Enable for unlimited sessions")
- Change checkbox value to 0 (for unlmited)
This will show the following on the form. If you check it, unlimited simultaneous sessions is set. If you leave it unchecked, the default of 4 is kept.
The result is the following (for checked and unchecked)