I will base my answer on the assumption that your supervisor is preparing to do something later that will impact the work of A, B, C, and D, and is seeking for feedback in order to accommodate their needs in this upcoming solution.
Matching inputs with others'
allows for superior — and as such, best-received — solutions
What needs to be considered is that A, B, C, D's individual interests may conflict with each other, or their priorities in aspects of a solution may differ.
A meeting held together could offer a chance to arrive at a solution that fulfills everyone's needs to the fullest — or to put it another way: it can help finding a solution that won't get challenged on the first day after its implementation.
Your first step could be to confirm whether this is indeed the case:
Confirmation step
You could approach A, B, C, D at first individually,
- introduce the case,
- reveal that the other supervisors are also stakeholders in the solution,
- and ask whether they deem a meeting together — in light of the above — to be justified.
(However, before you do this, you should probably confirm with your supervisor whether this is a step they approve of (just in case their intent was actually to collect information discretely, while not throwing too big waves in the organization — unlikely, but you loose a lot less by confirming than not confirming and making a mistake).)
If your superior approved this step, after polling the participants' preferences, go the path the majority votes on: meeting vs no-meeting.
Case of a draw
If there is a draw in the votes:
You yourself should not force a meeting in this case. Instead, you could go back to the meeting-proponents, and inform them that due to equal amount of declines, there won't be a meeting. Now the proponents could either put up with that (given it's not so terribly important for them), or, if it's important, then they could make a move to convince the decliners.
Resolution
If meeting:
If the final vote is for a meeting, organize a timeslot for it, and let the participants work out the feedback that your supervisor needs. (You could even offer your supervisor an invite to take part in the meeting, to observe the evolution of the final feedback first-hand.)
The timeslot challenge and a workaround
You wrote it could be hard to find a fitting timeslot. When you try to find a compromise in this regard, favor the needs of the meeting-proponents. If a meeting-decliner will end up not attending in person, it's way less of an issue (a non-issue, even, perhaps) than if a meeting-proponent gets left out.
In any case: if someone ends up not participating, offer them a chance to write up their needs that you will deliver in the meeting, thus representing their needs in their absence.
If no meeting:
If the majority of people expressed no interest in a meeting, you are free to collect input from everybody individually, and later present the results to your supervisor.
