Are you sure that all 4 were in your clan at the time of enlistment and that all 4 are truly in your clan right now? The best way to do this is to have them equip the true Clan's Clan Tag. If they are able to do that then they are actually in the clan. Sometimes the app or game will say they are in the clan but in actuality they are not. Doing this would solve that issue. If you find this is the case than look at #17 in this thread. Also, keep in mind I believe there is a short time delay between placement and the actual start of the war. It's possibly you didn't get the 3rd member until after the placement. They don't tell us the placement and they lead us to believe that the placement time is at the start of the war, but I'm not 100% convinced this is accurate.
Either way, if you truly believe you had 4 ahead of time then you need to contact @BeachheadStudio on Twitter right away (for the next time it happens...not now) and they should be able to get you placed if they feel they are at fault.