Is DDR4 RAM compatible in a DDR3 RAM slot?

At the Biostar booth there was a H170 motherboard and a B150 motherboard that had clearly defined DDR3 and DDR4 memory slots: The H170Z3 in

AMD B550 ATX DDR4 Wi-Fi 6E Motherboard - Black Model: B550TMHWKWIFI SKU: 6504286 MSI - B550 TOMAHAWK MAX WIFI AMD B550 ATX DDR4 Wi-

ddr4 vs ddr5 slot {{deEin Slot für DDR4 SO-DIMMs RAM auf einem Mainboard}} Source= {{own work}} Date= {{User:D-Kurumptdatedate=

ddr4 in ddr5 slot DDR4 DIMM Memory Slot part of motherboard, mainboard, system board, main circuit board, or baseboard main printed circuit of computer DDR