It's hard to come up with much with so little info, but that text Solarz posted does shed some light.
3 of the 4 deserters were newly inducted this year, with the 4th being a Corporal who joined in 2005. It was reported that the 4 were found en route to Liao Ning, which is where the Corporal is from originally. Seems there is little doubt that he is the ringleader.
The report goes on to say that when discovered by police, the deserters fired first, and in the ensuring firefight, 3 deserters and 2 policemen where killed, with another police officer wounded.
It was also reported that the 4 deserted between half 4 and 6 am, given the time, seems improbable that this was a spire of the moment thing, especially when you consider how much ammo was taken.
My guess is that this was pre-planned, and the 4 have been secretly stashing ammo in preparation, but must have failed to get into the armory to get more guns.
If this was pre-planned, then it only make sense that the men planned to do this when one of them was one sentry duty, which explains the sole Type 95, and how they where able to sneak off base without being noticed.
The police managed to take one of them alive, so the motives of the 4 should be known to the Chinese soon enough, although it is doubtful if they would decide to share that with the rest of us.
If I had to guess, I would think that this was either personal, or financially motivated.
If the Corporal had a tragic death in the family recently and the person responsible escaped prison, that might explain him wanted to go home for vengeance, and the others might have tagged along if they were good budds.
If money was the reason, they could have been planning to do a big bank job, or 4-8 military assault rifles with ammo could fetch a decent price on the black market all by themselves. If they were confident of gaining access to the armory, they might have set their sights on something heavier and more profitable even.