In case (4)
I believe the answer is:
lest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">in case</span>' is the definition.<br/ ><span class="explanation-format__explanation">(eg 'lest we forget')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lest that I've seen before include "To avoid the risk that - let's do it" , "For fear that (something should happen)" , "In order not to, say forget?" , "To avoid the risk that - we forget?" , "For fear that (something happens)" .)