I seriously agree with him...as I stated in an earlier post! Since your aquarium is 4' long it should get some type of vertical bracing in the middle areas. You can place this in the middle of the two doors and still keep things looking good. If nothing else use a 1x3 or so. You can even just glue them in place....no need to try screwing it in. It will only have weight on it vertically anyways! The way you have it built right now (or drawn anyways) it a "minimal" way of building it. There are definately commercially built tanks that use this method, but without using 2x4's at all (which is pretty much crap building) and I've seen how they start to fall apart over time. It may work fine, even for many years.....but then you just never know. Be safe! A couple of bucks and a few cuts gives big-time peace of mind!