The beard oil industry has done a good job of suggesting that the right product can fix a patchy beard. It can't. No topical product regrows follicles - oil conditions the skin and softens existing hair, which is worth something, but it won't create growth where your genetics didn't put any. Setting that expectation clearly is the most useful thing anyone can tell you about patchiness, because it shifts the question from "how do I fix it" to "how do I work with it."
Most patchy beards aren't as bad as the person wearing them thinks. A lot of what reads as patchy in the first few weeks is actually just underdeveloped - the hair hasn't grown long enough for the gaps to fill in visually. Four weeks feels like a long time, but most beards need closer to eight to twelve weeks before they show their actual shape and density. The first call is to wait longer than feels comfortable before making any decisions.
If you've grown it out and the patches are real, the fix is usually shaping around them rather than through them. A barber can clean up the lines in a way that draws attention to where the beard is strong and de-emphasizes where it's thin. This often means tightening the overall shape - defined cheek lines, a sharp neckline - which makes a medium-density beard read much fuller than the same hair left untrimmed.
Length choice matters too. There's a middle-ground length for many beard textures where patchiness is at its most visible - usually in that week-three to week-six range where the hair is long enough to expose gaps but not long enough to cover them. Going slightly longer, where strands can cross over thin spots, sometimes helps. Going shorter, where the density of short hair is more uniform, sometimes helps more.
For significant patchiness that doesn't respond to time or shaping, a clean shave or close-cropped stubble often looks sharper than a beard that's fighting itself. Some faces just suit clean lines, and there's no loss in that.
The goal is a beard that looks deliberate - whatever length that requires for your face.