Does this modem support 4-wire lease line operation?

Yes, the MT5600BA-V92 modem supports two-wire and four-wire leased lines. The four-wire leased line option includes dial backup and automatic leased line restoral features. Please note that, because leased-line operation consists of two client modems connected to each other, the maximum leased line data rate is 33.6K bps.

V.90 MODEMS:

The MT5600BA modem supports a two-wire leased line.
The MT5600BL modem supports a two-wire or four-wire leased line.