I'd use the longer 4-foot wide beds as this maximizes space. The only downside would be having to walk all the way around the 16-foot bed instead of having walk-ways between four-foot areas.

Here is a link that might be useful: BsnTech Gardening Blog

I can think of 2 ( more ) things to consider.

Will you have any plots south of your's ( shading some of your area ? ) or visa versa, will your tall growing or trellised crops be shading other plots north of you ?

I would also make sure that this is a revolving Plot, in other words, will you get to use this same area in future seasons AND that they will NOT be plowing the whole Community Garden area both in spring and in fall ( before you have finished your harvests, for instance )
I know some people that had planted rasberries, horseradish, walking onions etc ... and added amendments only to find that coordinators had rented 'their' plot to another gardener the following year.