I have never looked closely at a 4 seater, but if like the two seater, would guess that if you remove the spare wheel, you should see it between you and the fuel tank.It’s about 50mm diameter, clamped horizontally and may be coated with underseal.

Doug
2011 Plus 4 in Rich Maroon

1972 750 “ComDom” sprinter
1958 Triton 650
1992 Triumph Trophy 900