{{explanation}}
4 persons facing as for a reel of 4 up & down the room

1-8
: All Petronella turn (the set is now square), 1+2 change places RH
  (across the set); 1+3 cross diagonally LH, 1+4 turn 1/2 RH to end NHJ
  facing up, & 3+2 NHJ facing down
9-12
: All dance modified Set+link: All set. LH ends (3 & 4) turn 3/4 LH in
  centre **while** RH ends (2 & 1) cast 1/4 clockwise to form line
  across (L to R: 2 3 4 1).
13-16
: All dance 1/2 RSh reel of 4 across
17-24
: 1+4 also 3+2 turn 3/4 RH, All set facing up/down, on bar 20 turn to face
  across NHJ (now in same formation as bar 9 rotated 1/4 clockwise).
  Repeat Modified Set+link to form line up/down 2 3 4 1
25-32
: All 1/2 RSh reel of 4 up/down; 1+4 also 3+2 turn 1/2 RH, all set facing
  up/down 4 1 2 3