hey, is this your homework assignment?
it looks like a high side sense amp to me - for example, measuring current through a sense resistor tied to Vcc. and it does operate on negative feedback. mn4 is driven to cancel the extra current through mp2 - that's negative feedback.
basically, the circuit reflects the differential voltage at Vin to the resistor (provided that R1=R2=R3). Since the pmos mirror keeps the voltage at MP2's source equal to the voltage at MP1's source, the differential voltage must flow through the right hand resistor, R.
Additional current in MP2 raises the gate of MN3 until the current through MN4 cancels the extra current through the right hand resistor. Therefore, the current through MN4 is Vin/R and the voltage needed to create this current is (Vin/R)*Rout.
If the inputs are reversed, no output occurrs - sadly, this circuit is unipolar. gate of MN4 is pulled down until MN4 is off. you can reverse the pmos, and attach MN4:G to the other side if you want the other polarity.
Here is a little plot using R1=R2=R3=10k, and -100mV to +100mV sweep. As you can see, if VIN is negative we can't give any output, only 0v. but for positive input it's quite linear.
for an example application that needs high side sensing, see current limited hot swap controllers. but this is just one of many, many apps.