:

 HHHH
 HHHT
 HHTH
  HHTT 
 	
                 HTHH 
 	
                 HTHT 
 HTTH
 HTTT
 THHH
 THHT
 THTH
 THTT
 TTHH
 TTHT
 TTTH
 TTTT

There are 16 ways the tosses can come out.  The ways for which
there are two heads and two tails are HHTT, HTHT, HTTH, THHT,
THTH, and TTHH.  Since each way has the same probablity (namely
1/2 x 1/2 x 1/2 x 1/2 = 1/16), the chance that the four tosses
result in two heads and two tails is 6/16 = 3/8 = .375, or 37.5%.
(38.29% is what you get if you use the normal curve method of
approximating the chance.) 
 	! !I