Just a few things that stick out.
A Nano needs at least 6volt on V-in (input of the regulator) to produce a stable 5volt on it's 5volt pin.
An L293 is old technology, and drops typically 2.6volt in H-bridge mode at 600mA motor current.
That means you need at least a 6.6volt power supply for the L293D for 4volt on the motor.
Leo..