Thank you to everyone for the comments. These have helped.
The entire business of the meeting will focus on the adoption of one of four proposals, which will be submitted in writing before the committee meeting. The person to whom I assign the floor first will be allowed to move to adopt his or her proposal. Based on other comments above and my reading of RONR, the other members can then move to amend via substitution.Thus, if we have four total proposals, we would have one original main motion to consider the first proposal followed by votes to consider the other proposals through a series of motions to amend via substitution.
My real focus was how to assign the floor initially because I think the initial decision will be a big point of contention. In other words, all four members who submit written proposals will demand assignment of the floor. It makes sense to me to put the matter of assignment to an assembly vote, assigning the floor to the person receiving the largest number of votes (RONR p. 382). We are also likely going to have a bunch of subsidiary motions as well, including motions to table indefinitely. Thus, I am very concerned that we at least have some order to the initial part of the meeting so that the assembly has a clearer idea of what we are actually considering. As I would suspect many of you know, academic assemblies do not always follow Robert's Rules as closely as they should!