Question, concern PCIe PCIe

can i use m 2 pcie 4 0 in 3 0 slot   Dan candy burst pg slot

You're referring to NVME based M2 SSDs, correct? But yes if the SSD is PCIE spec then of course the speed will be less when used on PCIE

If your motherboard has a slot you are usually good to go You can use a SSD for both boot and storage If not, you can still use a NVME You can plug pcie into slots as it's backwards compatible but the speeds will be limited to Gen 3 It wouldn't be able to go above the

when was the slot machine invented Most computers and storage drives still use the common SATA or PCIe interfaces to sling your bits of data around, but if you're lucky SATA SSDs and SSDs that use two PCI Express lanes tend to use keys B and M to maximize compatibility, since both connectors can deliver both