I don't recommend 'stacking' the UCA. First of all, you will have to use WDM drivers in order to fully utilize all 4 UCA simultaneously; you cannot do that with ASIO since 'likes' to see only one device at a time. Granted that the UCA are all the same devices, but they are still using individual usb ports which, to asio, may count as one device per port. So, nada on asio with the UCA for simultaneous operation. One problem I have with WDM is that it tends to have higher latency than comparable asio, but this is dependent on the device and it's drivers.
One more thing, usb has a much higher cpu loading than comparable pci or firewire devices. By stacking 4 UCA's, you may be stressing the cpu more than by getting a single multi-input interface. Aside from that, the UCA is usb1.1 - much slower than usb2.0 or firewire. There are several multi-input usb2.0 devices such as edirol's UA-101 or MOTU's 828 usb interface.