A single legacy Mini DisplayPort monitor or other device of any kind may be connected directly or at the very end of the chain. Thunderbolt is interoperable with DP 1.1a compatible devices. When connected to a DP-compatible device, the Thunderbolt port can provide a native DisplayPort signal with four lanes of output data at no more than 5.4 Gbit/s per Thunderbolt lane. When connected to a Thunderbolt device, the per-lane data rate becomes 10 Gbit/s and the four Thunderbolt lanes are configured as two duplex lanes, each 10 Gbit/s comprising one lane of input and one lane of output.
Seems to me 4 lanes at 5.4Gbit would be enough to do 4k @ 60hz... Unfortunately, I don't have a displayport 4k monitor to test with.