let shiftRight1 arr = 

    match arr with

    | [|   |] 

    | [| _ |] -> arr

    | _       -> 

        let lastIndex = (Array.length arr) - 1

        let last = arr.[lastIndex]

        System.Array.Copy(arr, 0, arr, 1, lastIndex)

        arr.[0] <- last

        arr

​

[|1;2;3;4|] 

|> shiftRight1 

|> printfn "%A"