I'm not sure about the N/A rails, but S4 TII rails are already threaded. Some metric 1.5 pitch, don't recall exactly what.
But have a look at the N/A one's you've got, if the PD and FPR are threaded in and not stamped as part of the rail, just undo them and take a thread gauge to them and purchase the appropriate adapters.
Also, the tap's you've got should fit in the diameter, but there may not be much beef in the rails for a tapered thread if they're stamped pieces..........and if they're not stamped, you won't need to tap because they're already threaded
