I'm sure this can be done but I'm a bit unclear - if you picked Joe Bloggs and he was on sheets 1 and 3 how would you show both sheets when he was selected?
Or do you want the results summarised somewhere on the on the combo box sheet?
Do you want the extra column to be on the invidiual sheets or in the results when you search?
Presumably the combo box should contain a unique list of employees from the 4 sheets? i.e. if joe bloggs is on sheet 1 and sheet 3 he should appear in the combo box but only once.
In terms of flagging up the dates, presumably the date is a "Refresher Due" and if that is within 3 months of "today" then it is flagged? You don't necessarily need an additional column, you could use conditional formatting to highlight any cells in the relevant rows e.g. assuming your date is in the B2 cell you could add conditional formatting with a formula of:
=AND(B2 <= DATE(YEAR(TODAY()),MONTH(TODAY())+3,DAY(TODAY())),B2>=TODAY())
Come back on the bits and pieces above and I'll help you out.