Using interrupts for two MCP23017
The MCP23017 (and it's sister the SPI-based MCP23S17) can have their interrupt pins set to "mirror" mode. In this mode both pins are internally joined so when an interrupt occurs on any port both pins activate.  This is done using the MIRROR bit of the IOCON register.  Using this will turn your 4 interrupt pins into just 2, one for each Arduino INT pin.

Should you want to add more in the future, or if you find you want to use one of the interrupt pins on the Arduino for something else, there is also the ODR bit in the IOCON register. This sets the interrupt pins to be open drain, which means that all of them can be wired together into just one INT pin on the Arduino. Add a pullup (or enable the internal pullup) and you have have many IO expanders on one INT pin. Of course, that then means that you have to poll all the expanders to find which of them triggered the interrupt, so could slow things down somewhat depending on how you are using it.