Let employees in office i = Oi
Therefore : O1 + O2 +O3+ O4 = 6 where Oi>=1
Putting at least 1 employee in each office we are left with (Oi complement = oi)
o1 + o2 +o3+ o4 = 6 - 4 where oi>=0
o1 + o2 +o3+ o4 = 2
thus using combinations with repetition we have ( n +r -1 , r)
n=4, r=2
( 4 + 2 - 1 , 2 ) = (5,2) = 10 ways