It is not selfish to want your son to sleep through the night, its smart. God knows moms often feel guilty enough! My daughters both slept through the night early...The eldest, formula fed, slept through right at 11 weeks, the youngest, breast fed, slept through at 12 weeks. By slept through, I mean from 11 to 7. By 5-6 months, they were sleeping from 8pm to 7am. So I am one of those non-existant moms...Everyone says that I am just lucky or it's their temperaments...but they have completely different temperaments and I worked at training them. I will tell you that it is supremely easier to listen to them cry for 20 minutes now(though still hard, especially if they're your first), than hours on end as they get older...and as for separation anxiety, my daughters are more assured of themselves and loving than the three kids I know slept with their parents at night until they were 3, one of them 8! I don't know how long it would take to train starting at four months...but physically your child should be able to sleep throught the night without eating at that age. Falling asleep is a learned behavior...if you provide props(ie rocking, pacifier, a bottle, hold me mommy) to get them to fall asleep...they will want that when they wake in the middle of the night. And they do wake up, even though we call it "sleeping through the night," everyone has sleep cycles where they are in a deep sleep and a lighter sleep. Some babies even cry out during a lighter sleep, but normally not for long. If they learn to comfort themselves when they first fall asleep, they will more easily be able to comfort themselves in the middle of the night. I never let mine cry more than 20 minutes when I first put them down...because at that age, within 20 minutes they are asleep if they are fed, clean, and tired. If your child is eating enough during the day (at least 5-6 times) then they are just looking for attention during the night...train them that night time is not the time for play. If they have a wet diaper, change it and put them back to bed. I'm sorry to go on so long...but in response to your question...by about a month, my daughters were able to fall asleep with only the minimal amount of crying...unless they were over-stimulated...if they had not had any naps that day, believe it or not, they were more difficult to get to sleep at night! My oldest, who is almost three, takes a 2 hour nap in the afternoon, my youngest who is now almost a year, takes a 2 hour in the morning, one in the afternoon with her sister and (sometimes) a one hour nap in the evening (4:30-5:30-ish). If you ask any doctor, they will tell you that the amount of sleep children get will directly affect how they behave and eventually school work, etc. Babies need to sleep at least 10-11 hours at night (not including naps). Like my mama always said, that's when they grow!
I hope you are able to understand this...I know I tend to run-on and I hope you are successful. Please feel free to ask any questions...Good Luck!