With the dropping of LAX-RNO (4 flights) and LAX-SJC (8 flights), all that's really left in T3 is LAX-SFO and a few other markets like DEN and COS. With some creative gating, finding room for those 12 or so departures over in T4 shouldn't be too difficult.
Operating the split terminal doesn't make economic sense, in that you have to staff two ticket counters, two bagrooms, and have a lot more transfer drivers to move bags back and forth. It might be a bit crowded over in T4, but there's something to be said about getting more productivity out of the real estate...
It's the same with DFW Terminal B -- depeaking required fewer gates, so it makes no sense to be operating a third bagroom and a seventh ticket counter, not to mention all the transfer driver traffic between A, C and B.
As for the international flying and QF, NRT is a noon departure/10am arrival, QF is all evening departures and early morning arrivals. If the QF aircraft are remoted to the hangar to spend the day (which they typically would be with a 12+ hour ground time), there's plenty of room to gate QF and the new NRT trip without interfering with each other.