Disclaimer: These are my measurements and are NOT intended as manufacturing specifications or any specific brand.

When putting a 4150 carb on a 4500 intake, there is only 1/4" gasket sealing area as shown below (5.25-4.75)/2 = 0.25
That might be enough, but is minimal. And if using a gasket with the 4150 center hole, the gasket will not be supported above most of the intake plenum.
An adapter plate improves both of these. In addition, a 50cc accelerator pump body extends below the carb's baseplate, and can contact the intake without a spacer.