Although I'm okay doing it in many positions, I would have to say that I love having a guy shove his penis in my doggy style. With a 4 - 6 inch penis you should still be okay, but have you considered that this is also a great size (i.e. not too big) for perfectly good anal sex. So if you would like a tighter orifice to pump you could always try bending the gal over, putting her in doggy position, spreading the cheeks and gliding you rod in.
I have to agree with Franciz. 4-6 is about average, with 4 being just a little on the short side. I always enjoy missionary, where you can control the rate and depth of penetration. Any position will do. I have a tendency to get my woman all worked up before hand, so she is begging for penetration - any penetration.
There is no best position. Your anatomy will experience more pleasure with a particular partner in a particular position and another partner will be better in another. Very few people want to have sex in a single position for that matter.
With a 4 - 6 inch penis you should still be okay, but have you considered that this is also a great size (i.e. not too big) for perfectly good anal sex.
Maybe maybe not... ur forgetting girth size, and also (in my opinion obviously) i believe that that part of the body is an EXIT only... (with the exception for gays bc they kinda need it... no offence)
and missionary is usually what most ppl use... but (idr the name for it) i prefer using a doggy style type position but going into the vagina instead of the rectum.