Welcome Sam. Rotor Phasing. Drill a 3/8" hole in the top flat of the dist cap 2/3 of the way between the center terminal and the #1 cap terminal and shine your timing light above the cap straight down which'll in effect freeze the rotor like a strobe light on a dance floor at idle. If the rotor tip is too far away from the underside of the cap terminal it will miss as the spark cannot jump that distance. If using vacuum advance you want to duplicate the same amt of vacuum you have when it missed as vacuum advance will shift phasing clockwise (on a big block) and at 4500 is that at WOT or highway cruise (high vacuum). under load the ign system will tolerate less distance between rotor and cap terminal before missing but checking the distance at idle will get you started. If you have a Mityvac you can pump up the vac can at idle & note where the rotor moves. If there's a problem with the fuel pump pushrod worn down it could cause a delivery issue (will it miss at 4500 in your driveway or just at speed?). Best/easiest would be to tape a round vacuum gauge (most have a psi side) to your windshield & run neoprene line to the carb INLET & see how much pressure you have at the offending speed (& LOAD). Need at least ~3-4 psi under load. Could possibly have a vacuum leak & causing a lean miss at 4500. Not sure how to check for that without a LM1 meter or similar but I myself might try installing a junk power valve that's been gutted to enrich the mixture or possibly removing it just for a quick test. Holler with any news