Basic explanation of how to use arrays in a datastore?

First things first you CANNOT use ordered datastores with arrays! Instead use a normal Datastore:

https://www.robloxdev.com/articles/Data-store

Next, I hope you are familiar with saving and getting data. Here is a simple script that saves and loads for you :)

local datastoreService = game:GetService("DataStoreService")
local datastore = datastoreService:GetDataStore("LeaderstatsSaveSystem")

--Load in data
game.Players.PlayerAdded:connect(function(plr)
	--Get stored data
	local MainSave = datastore:GetAsync(plr.UserId)
	--CHANGE THE FOLLOWING FOR YOUR LEADERSTATS
	local currencyName1 = plr.Leaderstats.currencyName1
	local currencyName2 = plr.Leaderstats.currencyName2
	local currencyName3 = plr.Leaderstats.currencyName3
	local currencyName4 = plr.Leaderstats.currencyName4
	if MainSave then --checks if the player has saved data
		currencyName1.Value = MainSave[1] --sets the value to the saved data
		currencyName2.Value = MainSave[2]
		currencyName3.Value = MainSave[3]
		currencyName4.Value = MainSave[4]
	end
end)

--Save on leave
game.Players.PlayerRemoving:Connect(function(plr)
	--CHANGE THE FOLLOWING FOR YOUR LEADERSTATS
	local currencyName1 = plr.Leaderstats.currencyName1
	local currencyName2 = plr.Leaderstats.currencyName2
	local currencyName3 = plr.Leaderstats.currencyName3
	local currencyName4 = plr.Leaderstats.currencyName4
	local Save = {currencyName1, currencyName2, currencyName3, currencyName4}
	datastore:SetAsync(plr.UserId, Save)
end)

You are Welcome :)

PS: If this works please accept as answer.