‘Between ourselves’ tests you on prepositions.
Prepositions are such handy and vital words ... yet speakers of various languages use them surprisingly differently.
In French, for instance, you would (quite logically) come in from 'under' the rain and get 'under' a warm shower, whereas in English we would more naturally say 'in from the rain (or 'out of the rain'), and into a shower'.
English also says 'between ourselves' (as does the French, 'entre nous'), while German idiomatically uses 'unter uns' ( = 'under us').
So you do have to watch carefully how English does such things!
In Answer 1, 'dealing with' would have been acceptable; Answer 4's double use of 'watching' does not quite work, because she would (probably, more likely) have been watching outside, or through, the window rather than watching the window itself. (One can 'watch the door', i.e. 'keep an eye in case anyone comes', and famously, on the London Underground and elsewhere one is encouraged to 'Mind The Gap'.) One might also 'watch over' a sleeping child, pet or someone that was unwell (like the shepherds on the hills in the Christmas story), but 'watching over' a range of documents does not quite feel natural or appropriate.