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

1-8
: 1s & 4s cross down/up 1 place & cast back to ends to meet partners in
  centre in prom hold, 1s+4s dance RSh round each other & face up/down
9-16
: 1s dance RSh reel of 3 across with 2s **while** 4s dance reel across
  with 3s, 1s & 4s dance back to original places **while** 2s & 3s dance
  into middle
17-24
: 2s+3s dance RH across & LH back to end facing corner persons
25-32
: All set, change places LH & 1s+4s circle left to end 2 1 (4) (3)
33-40
: 1s+4s dance 1/2 R&L & all set twice to partners. 2 4 (1) (3)