Hi, It is really complicated to imagine justifying any kind of device to make 4 plates per month but...
Least expencive would be HP LaserJet 5000 or 5100 with any laser poly-plates. We had used this set-up for a really long time with good resaults. Printer/Cartridge must be kept at excellent to great condition (no room for OK) and properly set would give 3-5K implressions with halftones and 12K+ with just solids. We actually use Xante laser plates. Lately even with better resaults we use OKI ES-3640 machine which is same engine as C9800 and Xante Ilumina. But like I mentioned above, even nice HP 5100 will cost about $500-600 for 4 plates per month... Spending even this little, I am not sure it make sence.
If you deside to go for it anyway, let me know, I will share a couple of Must-do tips.