Can I go from 105-fg4000 or is that to big of a jump. I am wondering if not doing 205 in the middle would make a difference ?
To my mind, trying to find an absolute answer regarding a specific combo is an exercise in futility.... paint hardness varies car to car, the depth of the defects vary car to car, peoples polishing techniques vary, the way products react in different environments come into play etc etc.
Instead, the way I approach every car I detail is to go into it without any pre-conceived ideas about how they paint will react to a certain combo, and start the process of my testing with the least aggressive combo. As you then increase the aggressiveness of your combo for your test spots, you will be removing be removing more defects and at the same time leaving more 'haze' in the paint.... the more aggressive the initial cut (and therefore the haze), the more likely you will need to come back and perform the refining polish. Making the call about if the refining polish is necessary is going to depend on the results you see from your test spot, the customer's expectations in terms of the level of defect removal and gloss, and if they are willing to pay for the 2nd round of polishing.
I have done a 1 step using SF4000, a 1 step using PF2300 and a 1 step using FG400, all on various VW paint, depending on the condition of the car I was working on. Likewise, I have had to do initial cuts with FG400 and followed them with PF2300, and have done initial cuts with FG400 and followed them with SF4000.
There are just so many variables at play that trying to 'copy' what worked for detailer x on a certain car may not deliver the results you are looking for on the same car.
So as others have mentioned, conducting test spot(s) is really the only way to know what will work on that car, for that customer, on that day.
Let the paint guide you.