I need to make 4 minecraft accounts ( which I know are free.) but I need to download/buy the game for each device. what's the cost to buy bedrock? (Is it monthly? yearly? once only?)
Minecraft is a one-time purchase, so there are no recurring (monthly or yearly) costs.
However, Minecraft does include a marketplace, where people can purchase various skin packs, worlds, and so on. Some of these are free, but many require so-called "Minecoins", which must be bought separately with real money. These packs are entirely optional, however, and Minecraft can be experienced fully without spending a single Minecoin.
As for the exact cost, that depends on the store you choose to buy it. If you choose to buy the game digitally, its price will also depend on the region in which you buy.