I know exactly what you mean! My husband is the kind who will try to do what seems best for me (though without asking my input), meaning that he won't ask for sex when he thinks I'm too tired. That made for very little of it during the first year of my second child's life--I was always tired! However, I recently had a serious talk with him and told him not to assume that I was too tired, that sometimes I just needed to be asked, that I could always take a nap if I had to, etc (I don't work outside the home).
Okay, you said the idea of scheduling it takes away some of the pleasure. That may be. But think of it this way. Men have a very physical need for sex. It's has to do with their physical make-up. When we help them meet that need, it strengthens the marriage bond, at least from their perspective. You said the idea of scheduling take`s away some of the pleasure. Is that from experience or is it just what you think will happen? Sure, before you have kids, the spontaneity adds to the thrill. But once you've had kids, scheduled sex is much more thrilling than none at all!
Yes, sex will be different after having kids. For me, I've found that it is actually better. I have more reasons to love my husband (a girl and a boy so far), and also have built up more creativity over time. I think back to our first year of marriage... it was nothing compared to what we have now.
The truth is, where you're at with a 4 month old (who may or may not be waking up still at night) is not where you'll be forever. Babies do eventually sleep through the night. If you're planning on stopping with two, then things will only get better. Eventually both kids will go to bed at a decent hour and you'll have time to have some "fun," as we call it, before you hit the sack. You'll be able to be more spontaneous again.
This would never work for us, but some people find waking up in the wee hours--like 1:00 or 2:00 am--to have a little romance and then going back to sleep works for them. This tends to be rather spontaneous, and would have to work for both of them.
Another thing. Look at your evenings. What exactly do you have to do each evening? If you stay home, what can be done earlier? If you work, that would be harder. But make a list and get things done--don't get distracted with little non-essentials like the Internet. Encourage your husband to do the same. Once the kids are down for the evening, you can decide if you want a little romance, or just to read or even go to sleep early. This of course won't happen every night with a baby--I know a couple who would sometimes interrupt romance to nurse a baby, but would continue when it was done--but focusing on a strict evening routine could help, and more as the baby gets older. HTH!