While Lion miter machines using compound-lever knives are seen most often in professional picture framing and cabinet shops, another handy miter machine is the old H.C. Marsh, later made by Stanley. It'll handle 4"-wide stock, and also serve as a miter clamp.
It cuts the initial miters and/or fine-tunes the fit by cutting both simultaneously.
And it then rotates 90 degrees for fastening.
Not as fast as a Lion knife, but with other handy features.