If a player is not in my group, I want it to say customer How can I fix this?

You can use the player's IsInGroup() function

if(not Player:IsInGroup(4731512))then
    Rank.Value = "Customer"
end

Remove else and did you mean GetRankInGroup? GetRoleInGroup returns a string.

local function PlayerAdded(Player)
    local stats = Instance.new("IntValue")
    stats.Name = "leaderstats"
    stats.Value = 0

    local Rank = Instance.new("StringValue")
    Rank.Name = "Rank"  
    Rank.Value = Player:IsInGroup(4731512) and 'Customer' or 'Guest'

    Rank.Parent = stats
    stats.Parent = Player
end

game.Players.PlayerAdded:Connect(function(Player)
    PlayerAdded(Player)
end)

for _, Player in  pairs (game.Players:GetPlayers()) do
    PlayerAdded(Player)
end