F# : Project Euler 2 solution in 4 different ways

gistfile1.fs

module Program
open System

let problem2 = // 4613732
  let mutable n1,n2,sum=1,2,0
  while n1<4000000 do
    if n1%2=0 then 
      sum <- sum + n1
    n2 <- n1+n2
    n1 <- n2-n1
  sum

let problem2_2 = // 4613732
  Seq.unfold(fun(n1,n2)-> Some(n1,(n2,n1+n2)) )(1,2)
  |> Seq.takeWhile((>)4000000)
  |> Seq.filter(fun n->n%2=0)
  |> Seq.sum


#nowarn "40"
let problem2_3 = 
  let rec fib =
    seq{
      yield! [1;2]
      yield! Seq.map2(+) fib (Seq.skip 1 fib) }
  fib
  |> Seq.filter(fun n->n%2=0)
  |> Seq.takeWhile((>)4000000)
  |> Seq.sum

let problem2_4 = 
  let rec fibList=function
    | n2 :: n1 :: _ as l when n2<4000000 -> f <| n1+n2 :: l
    | l -> l
  fibList [3;2;1] |> List.sum

[<EntryPoint>]
let main _ =
  printfn "%d" problem2
  printfn "%d" problem2_2
  printfn "%d" problem2_3
  printfn "%A" problem2_4
  Console.Read() |> ignore
  0