From the Nordic components website, under "additional information"....
I believe that the standard Versamax tactical is coming with the +5 extension (8+1 capacity, 3 standard with a +5), so if you're going 4" longer, based on the info below, you're looking at a +7. If you already have one, just figure out what you need in extra length.
+3B – Adds 9.5 inches. For Benelli M1/M2 Tactical, +9 – Adds 22.5 to 22.9 inches, +8 – Adds 19.8 to 20.2 inches, +7 – Adds 17.1 to 17.3 inches, +6 – Adds 14 to 14.4 inches, +5 – Adds 11.6 to 12 inches, +4 – Adds 10.1 to 10.5 inches, +3 – Adds 8.3 to 8.7 inches, +2 – Adds 5.7 to 6.2 inches, +1 – Adds 3.1 to 3.5 inches