I've directly compared my GT3541LS against a GT3530LS with my 500/4 working 850mm. I didn't see enough of a difference to change my mind.
The G-Lock leg collars seem to work and seem to do the job with that 4 section.
If you are comfortable with the 3 section then you should get that. Don't worry about us here .
In the world at large I see very few GT3541LS's in the field, where what I see is mostly holding up super-teles. The GT3532LS traces its lineage back to the G1325, which has always been the workhorse wildlife lens. I don't think that has changed in the world at large.
As Joe mentioned, there are portability benefits, which is why I bought mine.
Back in the days of the G1325 there was nothing similar to the GT3542LS. A tripod buyer was forced to choose between a Series 2 and the G1325. That entailed a significant compromise between portability and stability.
My GT3541LS gives me the best of both worlds. I bought it primarily to replace my 4 section series 2. I found it works well enough with my 500/4 that I ended up "stealing" it for that use.
If I saw little value in the shorter 21.7" folded length I would go with the 3 section. Certainly can't hurt and the setup/take down time is slightly quicker.
One rarely discussed consideration would be if you sit on the ground and shoot. In that case, you cannot lower the working height with all legs retracted without going to the next leg lock position, which drops the camera quite a bit. I think the 4 section here provides more flexibility because it can go lower before needing to change lock angle positions. From there it can be precisely positioned by extending one section a bit.
The value of that depends on your height and whatnot, and exactly how you might go about shooting that position. The 3 section may work just as well, or maybe not depending on preferences. The 4 section works well for me in that way and I do shoot that way quite a bit.