As state-of-the-art fabrication techniques are approaching the 3 nm size, the traditional silicon-based circuit faces huge challenges. Transistors based on two-dimensional (2D) materials have attracted much attention as potential alternative candidates. However, critical performances including the subthreshold swing (SS), on/off ratio, and magnitude of the on-state current for 2D transistors around 3 nm size are far less to be studied well. In this work, we propose in-plane Schottky-barrier field-effect transistors (SBFETs) with a 4-nm channel based on the lateral heterostructure of monolayer 1T/2H MoTe2 and WTe2. The electric transport properties are investigated by first-principles quantum transport simulations. At a 0.64 V bias, the WTe2 SBFET has an on-state current of 3861 μA/μm, with a 4.5 × 104 on/off ratio and an SS of 87 mV/dec, while the MoTe2 SBFET has an on-state current of 1480 μA/μm, with a large on/off rate of 3.6 × 105 and an SS of 78 mV/dec. Our results suggest that FETs based on the lateral heterostructure of 1T/2H MoTe2 (WTe2) are promising candidates for high-performance 2D transistors.