The sliders are fixed in function. They do assume different modes (sometimes controlling each PART - sometimes controlling each Element/Operator) but each slider has a fixed mapping for each respective mode. There's not a way to directly make slider 4 control PARTs 4+5 together.

The conventional solution would be to utilize a knob. Either superknob or your assignable knobs. You can have one knob control the level of multiple PARTs at once.

I know, it's not a slider -- and I personally have more issues using knobs for volume on the bandstand than sliders. So this may not be preferable. Or it may work fine for you.

Keep in mind you can have an expression pedal control superknob. So your foot can act as the volume control of more than one PART at a time. And this can be assigned differently for each Performance.

A less conventional solution would be to indirectly link a slider to a PART's level. This approach allows for one slider to control multiple PARTs' volumes at once. However, for each slider you want to assign to control other PARTs - you have to "burn" a PART. You can't have the slider control its own PART plus other PART(s). You have to dedicate the controlling slider as control only and the PART associated with the controlling slider will not make any audible sound at all.

Say you want slider 2 to control PARTs 3 and 4. What you would do is go to PART 2 and I would make this PART FM-X (not AWM2). Set the output of PART 2 to OFF (the default is Main L/R). Then you would make some changes so PART 2 (internally) generates a single-note "drone" that continually sounds from the time you press the first key and continues to ring out "forever". I go into more detail here: https://yamahasynth.com/ask-a-question/volume-parameter#reply-74248 ... also read the other messages there as there's some other approaches mentioned in the thread.

PARTs 3 and 4 would use the control matrix to set the source controller as envelope follower 2 (follow the output level of PART 2) and the destination as the PART's level. As you move slider 2 - your volumes of PARTs 3 and 4 will be offset (up and down as you move the slider up/down). There's a more significant amount of programming involved here -- and it's not as intuitive as more direct knob assignment. But it allows you, at the expense of a PART per "multi-PART-control-slider" to have a single slider control more than one PART at a time.