Optimal Leading Card in Cribbage
Having spent some time trying to work out probabilities and carrying out some research, I have come up with a few conclusions. 


Almost NEVER lead with a 5 or a 10.


There are 16 cards that value 10 in the pack (30%), so playing a 5 gives a very high probability that the other player will get 2points for 15.
Same theory for leading with a 10. Although far fewer 5s are in the pack, these are highly valuable cards and often not sent to the crib. 

Keep back Aces for the end


They come in very useful for getting 31 for 2, and possible doubles or triples.

6s, 7s, 8s, 9s


These are strong cards in a hand, so are often quite easy to 15. Leading with a 9 or 6 allows the other player to get a 15 with little worry of a run occurring. 7s and 8s can trick the other player into going for a 15 to gain you a run, but can be followed by another card to make a further run from the other player. This would give the other player 7 points and you 3 points. Not a great return.

Deuces and Treys


2s and 3s are what is left. A 2 is probably useful to hold back for the same reason as the Aces, but 2s and 3s are the safest options for leading.



So, the answer to vary the leading card (i.e. instead of always playing a 4) is to use a 3, or even possibly a 2.