Functions don't get used with that particular syntax which is probably why you're having trouble. EXEC is for a stored procedure.
Try -
select 1, 2 from x
union
Select [3],[4] from dbo.fnc_function1()
Keep in mind all of the usual rules about union (same columns, in the same order with the same types) applies here.
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?