reduce from 12 to 7.5 volts and can carry 4 ampere
The L296 from STMicroelectronics has the following specs:

Voltage - Input (Min)   9V
Voltage - Input (Max)   46V
Voltage - Output (Min)  5.1V
Voltage - Output (Max)  40V
Current - Output    4A


which appears to meet your needs.  Available at Digi-Key for $5.60.