{{explanation}}
3s & 4s start on opposite sides

1-4
: All set then 1s & 4s 1/2 turn RH moving to centre into prom hold as 2s &
  3s cast to ends
5-8
: 1s & 4s in prom hold dance 1/2 reels of 3 across with 2s/3s (1s RSh to
  2M, 4s RSh to 3M)
9-16
: 1s+4s dance RH across 1/2 way & 1/2 turn partner into prom hold, 4s & 1s
  prom hold 1/2 reels of 3 across with 2s/3s (4s RSh to 2M, 1s RSh to
  3M)
17-24
: 4s+1s dance RH across 1/2 way & 1/2 turn partner into prom hold, 1s & 4s
  dance 1/2 reels of 3 on sides (4s RSh to 2M, 1s RSh to 3M)
25-32
: 4s & 1s dance into 1/2 reels of 3 across (4s LSh to 3M, 1s LSh to 2M),
  4s+1s dance 1/2 RH across & cross RH to end 3 1 (4)(2)
33-40
: All circle 8H round & back