At 4 months it isn't usually sex, it's usually a dominance thing. If he feels he already is dominate over Paris he won't bother but the other chi was a new factor and this was his way of showing he was alpha. There is another thread about this in Chi Chat I think. Marcus was lifting his leg at 4 months too so evidently that isn't too young.
Ah found the thread if I can make the link work
http://chihuahua-people.com/viewtopic.php?t=5714
Ah found the thread if I can make the link work
http://chihuahua-people.com/viewtopic.php?t=5714