I have a 12 V, 1.25 A supply and want to make it 4 V, 2 A using a voltage divider design. What method should I use for this design?
I assume the term "voltage divider" is a vocabulary mismatch, what you need is a buck DC-DC converter to get 4V at good efficiency.
Here is a list of candidates (I filtered on current and voltage).
Here's a nice cheap one.
I picked one with adjustable output because 4V is not a common output voltage, so it is not available in fixed output voltage version.
You can design one if you want but if you only need one to solve a problem, readymade ones are faster to implement.
That won't work.
Voltage dividers reduce predictably the voltage of signals, not the voltage of power supplies.
You need a switching DC-DC converter (step-down a.k.a. buck regulator) instead.