what prevents a ddr3 dimm from being installed in a ddr4 dimm slot on a motherboard?

A DDR3 DIMM cannot be installed in a DDR4 DIMM slot on a motherboard primarily due to physical and electrical differences. Physically, DDR3 and DDR4 modules have different numbers of pins (DDR3 has 240 pins while DDR4 has 288 pins) and different key notch positions on the connector. These differences prevent the DDR3 module from fitting into the DDR4 slot. Electrically, DDR4 operates at a lower voltage (1.2V) compared to DDR3 (1.5V or 1.35V), and the signaling and timing characteristics differ, making them incompatible even if physically it were possible. Thus, the slot design, pin count, key notch placement, voltage, and timing differences collectively prevent DDR3 DIMMs from being installed in DDR4 slots.