Yes that should work, although if there are significant variations in Vf some XPLs will get more current than the others. Also I would warn that a large PSU could sabotage the efficiency of your build in several ways. Some have very low efficiency at low loads and it may have a significant idle power consumption just by being plugged in. Supposedly the newer power supplies behave better, maintaining a ~70% efficiency even with a low load but I have not tested this.
4 XPLs at 1.3A would be 17W of dissipation. Depending on the size of your PC case, that could be enough intensity for flowering good sized nugs. If the light is too bright, it encourages the cuttings to try and respirate and photosynthesize and without roots that can cause them to wilt. When growers see the wilt they try to make up for it by increasing humidity and decreasing air flow, which can lead to rotting. The best way to make cuttings in my opinion is to use dim but direct light. That keeps them from trying to flower, but also keep them from trying to grow too much. That way they will not wilt, will not rot and do not need very high humidity to maintain turgidity. For whatever reason it seems like the plants that maintain turgidity have a better chance of rooting and taking off.
I think I have a driver that puts out 900mA and have a low Vf range I will try to track it down. If you used a pair of XPLs in series at 900mA, that would be very high efficiency, cheap and should be plenty bright for cloning.
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