4 month old will only cry himself to sleep

My 4 month old son will only fall asleep by "crying it out". I have tried sleep training him, but the crying is supposed to stop after a few days, and its been a month without any change. While sometimes he will fall asleep after only 5 minutes of crying in his crib, and others after an hour (at which I go in his room every couple of minutes to reassure him, but don´t pick him up unless absolutely necessary), he generally cries for about 20 minutes and then falls asleep (both at night and for night time). Even if he is falling asleep, the moment i place him in his crib he starts wailing, and will do so until he falls asleep. On the other hand, he does not fall asleep in my arms either. It seems the only way is for him to have a little tantrum (sometimes very little, sometimes very big) and then he falls asleep. Other than this, he is a very healthy baby. It  may be relevant to mention that he sometimes uses his thumb to calm himself and when he falls asleep (he refuses a pacifier). I have a bed time routine for him, put him to bed at 8:30 pm every night and have made his crib as comfortable as possible (although this behavior holds true even when we sleep away from home and in other cribs). Please help! I wan´t my baby to fall asleep without having to scream his head off!

[Kevin Kennedy, Ph.D.]

He is behaving in an entirely normal fashion. At this point you are going into his room too often,.Space out the intervals bit by bit. By going in so often you are reinforcing the crying.