I don't think it's sexual at this age. I think it's just part of expression and exploration. At our daycare, my 3 year old is learning to go potty and the bathroom is shared and open so that the teachers can see what's going on in there. Boys and girls go together and they do notice other people's bodies.
My son has recently started to point out his little brother's "wee-wee" in the tub. I just respond, "Yes, that's Ryan's wee-wee and you have a wee-wee, too." He will sometimes want to touch Ryan's and will say, "Touch Ryan's wee-wee." I just tell him, "No. You have your own wee-wee that you can touch. We don't touch other people's private parts." He's 3, so I try to be as basic and simple as I can.
Your son is 4, so probably has a better grasp and understanding. I would just CASUALLY, mention that it's not acceptable to pull down your pants unless you are going potty. Making a huge deal about it will probably just make it worse. But, I often find myself being rather "matter-of-fact" in my parenting style, so others may not agree.
As for the kissing, I agree with MR's post. That's probably how you show affection (Gosh, I kiss my boys 100 times a day and I ask for kisses ALL the time from both of them.) It's just how we show affection and your son doesn't mean anything by it.
One thing that we have stressed with our son(s) is that he doesn't have to give hugs or kisses to people if he doesn't want to (relatives for example). I like to give him the freedom to say "No thank you." Besides the obvious reason of giving him this right, I also can turn it around and tell him how he shouldn't give hugs or kisses to people that don't want them.
So, my long-winded piece of advice is to just calmly talk with your son and explain the importance of privacy and touching.
Good luck.