Two typical ways of doing this if there is nothing commercially available.A low drop out linear regulator is pushing it, a 0.8V dropout voltage at 4 amps is a tough ask, and at 25V your power dissipation is starting to climb (24Watts). Look at the LT1084 as an example.Many LDO regulators have a tendency to be unstable, you have to read the data sheets carefully, and heatsinking must be taken care of as well. They are not efficient, and for that reason, another method would usually be used.
A buck (step down) switch mode power supply(SMPS) is likely to be the answer, but these are not things that you can design without a lot of reading and experience. There are often demo boards available from vendors of SMPS chips that may, with some modification, do what you want. You can find these by going to places like Digikey, Mouser, Element 14, or the chip vendors themselves.The chip vendors, Analog Devices, Maxim, Texas Instruments, for example (there are many others) often have provision for entering your basic specifications, and they throw out a list of suitable SMPS chips. From there you have to narrow down your search, and then of course you have to design the thing and do a board layout, hence my comment about experience earlier.Cheers,Richard
