What is the difference between PCIe Gen 3 and PCIe Gen 4?

What is DIMM and SO-DIMM Slot? ram slot 2 and 4 8GB LPDDR4x Virtual RAM Expansion: Supports 3 expansion options, 8GB + 468GB 2×2 MIMO, Supports up to UL Cat 13 DL Cat 12 ram slot 2 not working : 2 ram slot motherboard FRONTECH H61 Chipset Motherboard with 2xDDR3 RAM Slots LGA1155 Supports i3i5i7Pentium Processors 6+4 USB Ports

rama slot Should RAM be in 1 and 3 or 2 and 4? There is no difference in speed between these two pairs, but it is often suggested that slots 2 and 4