If you don't have an MFT you'll have to set up so the rail is supported and you can accurately set the rail to finish the cut after you flip the piece end to end.
Do you have a planer or a hand plane to square the face once you do the second rip on the flip side of the board? Even with super precise placement on the flip side, you're liable to be off by a few thousandsths of an inch, and have a faceted face on the ripped side.
I'd take a piece of scrap ply say 40" put a couple of tacks though it from the bottem, attach the 4x4 to it off Center. Clamp the ply down/ secure it. Set your rail on it, build up the outboard side of the rail to support it . Rip the 4x4 flip it and repeat.
I know you want to use your TS75, but to be honest I think the right tool for ripping something like this is a table saw or band saw, followed by a pass on a jointer to flatten up the cut face.
If you really want to (or need to) use the TS75 to resaw a 4x4, then I agree with the idea of putting a second piece of wood of equal height next to the 4x4 to support the track. Cut half way through. Flip piece over. Raise saw 1-2 mm, and repeat. This will result in a slight leftover middle strand between the two halves, which creates a bit of a spacer and reduces the chance of the saw binding up on the second pass. Split or pry the two pieces apart, scrape off the middle strand, and joint the resawn faces.
I've done it with a portable table saw referencing the same face and cutting through half way on each side - slightly deeper on the second pass. It's never perfect, but close then can be planed. Running a skillsaw freehand is the least accurate. I don't own a TS -75 so I don't know depth of cut? I'm assuming it's less than 3 1/2" US. In the UK a 4x4 is 3.75" actual?
Just wondering if you could bring it to a lumber yard and let them Make one pass with either a bandsaw or tablesaw for $2.
In the U. S., 4x4s are about 3.25 inches square. They can be ripped on a decent table saw. A ripping blade is recommended. My little Ryobi BT3100 can and has done it. Ripping pieces narrower than the track on a track saw is possible but much harder than ripping them on even a small table saw.
Domino and glue two pieces of 2x4 perpendicular to the 2x4 you are ripping (12" will do for each of the scraps). Now you can clamp the scrap pieces to the MFT, and lay the track, with the scrap supporting the track as well. Rip with ease.