4v4 would work better than 5v5
There's mainly two reasons for this imo:
From a gameplay perspective, this game needs either space or at least the feeling of space to work, when things get congested by people playing intelligently as a unit it falls apart worse than other football games. Naturally if it's 4v4 then you have more space so the problem of 5 players congesting their own box is less of an issue.
From a matchmaking perspective, 4v4 means less players are required for each individual lobby, which means they can introduce more stringent matchmaking criteria (tighter SBMM, better server locations etc.) or have faster or more varied matchmaking, both of which will naturally make it a better experience. With the size of the playerbase I think this is very important for the long term viability of the mode.