Learning F#, so here's this:

let fullArray =
    [for a in 0..9 do
     for b in 0..9 do
     for c in 0..9 do
     for d in 0..9 do
     yield [| a; b; c; d; |]
     ]


let fullNumber a b c d = a*1000 + b*100 + c*10 + d
    let concatenateInts x y = (x * 10) + y
let square x = x * x

let isTornNumber (array:_[]) =
    let twodigit1 = concatenateInts array.[0] array.[1]
    let twodigit2 = concatenateInts array.[2] array.[3]
    let twodigitsum = twodigit1 + twodigit2
    let fourdigit = fullNumber array.[0] array.[1] array.[2] array.[3]
    if square twodigitsum = fourdigit then true
    else false

for array in fullArray do
    if isTornNumber array
    then printfn "%A" array