Looking at the BEDS numbers from 2012-13 to 2014-15...
School.....+/-.....Overall difference from 700(4A threshold)
WDM Valley.....+160.....+1496
DM East.....+34.....+1041
DM Lincoln.....+70.....+959
Dav West.....-126.....+695
SE Polk.....-1.....+810
IC High/IC West.....+221.....+2027
Waukee.....+236.....+958
Linn-Mar.....+133.....+841
Johnston.....+106.....+825
What I gather from these numbers...
== West Des Moines could definitely split and still be comfortably near the top. Valley is over triple the 700 minimum for 4A. But it doesn't seem like they have any desires to do so.
== DM East, DM Lincoln and Dav West are all metropolitan schools. Despite their size, seems very unlikely a new school would be added. In fact, redistricting could take from their numbers some and supply DM North and DM Hoover - though, the likelihood of that is in question also.
== SE Polk could split and probably form two 4A entities like Ankeny did. Not sure if a new school in that area would draw from some of DM East's pool.
== Iowa City is splitting. Growth in the North Liberty area is faster than anywhere in the state, save for perhaps eastern Dallas County/western Polk County. The new school should almost certainly open as a 4A entity, but if not, would probably be so within one cycle of districting.
== Waukee is exploding and a split seems feasible, with a possibility of both being 4A also.
== Marion(Linn-Mar) seems to be in a similar place as Waukee and Iowa City. Some redistricting could take place and possibly boost the numbers at Marion Independent, but that would take a substantial give. And I'm uncertain if Marion Independent could handle that kind of additional influx.
== Johnston looks like they're close to split, but not quite as far along as the other suburban areas.
4A could conceivably add anywhere from 4 or 5 schools in the next decade or so. Unless of course the state creates a new 5A, then we'll just have the extra schools at the top, while closures and consolidation take from the bottom.