First of all, do not rush into ANYTHING with this woman. And I'm really concerned with the way you describe your "desires" referring to your sexual needs, although you're still a virgin. You make it sound as though your main focus will be to have sex with this woman 24/7, and you wonder how you can be a good influence to her 4 year old son, while you're having all of that supposed sex with his mother. That is very concerning to me, because you're already talking about marriage, before you've even slept with this woman yet.

Don't even think about marriage or the boundaries and responsibilities that go with being a step-parent, until you've dated this woman to the point where she trusts you enough to be around her son, and where you both are ready to discuss marriage.

Don't put the cart before the horse. If you do, your cart will go nowhere, and your horse will just stand there and starve to death.