I was always taught to retain hands. But think about the "flow of the dance" - there is no logical reason to make the awkward move to release hands and then rejoin.
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From: Jim Healy [mailto:email@example.com]
Sent: Thursday, May 13, 2010 6:20 AM
To: Strathspey List
Subject: RE: Tournée - end of bar 4
Shocked and stunned that Sophie does not have a manual to hand at work. Tut! Tut! ;o)
However, what it says for bars 2-4 is:
2–4 Both couples dance round half-way, anti-clockwise, the two men turning their partners across in front of them on the fourth step so that all are in a line up and down the middle of the set, the women facing towards the man’s side and the men facing towards the woman’s side of the dance.
So, no answer to Wouter's original question but I have always taught (and been taught) that both couples release one hand and retain the working hand (right at the top; left at the bottom) ready to start the turns.