There's no way to predict that, sorry. Each child is different.
Bear in mind, that your daughter's situation, may not necessarily mean it will be duplicated in your son.
At 4 months... they often 'find' self-soothing through sucking, fingers or pacifiers. It's common.
My son did that too.
How you go about it is up to you. But, I personally find nothing 'wrong' with self-soothing. Its a normal natural instinct in babies. EVEN in the womb, they do this. Many child experts will also say that self-soothing is a GOOD thing... in that it shows self-motivation in a baby and that they are doing what comes naturally and it contributes to a nurtured confident baby/child. Nothing wrong with it.
Or, you can try and give him a 'lovey' or something to cuddle with, instead.
Also, if he does wake at night, being he is only 4 months old, this could also just 'signal' that he is having a growth-spurt, and needs to feed, as their intake needs are increasing AND they do just get hungry. Thus, he is sucking his fingers...because THIS is also a "hunger cue" in babies. ie: rooting around for food. It's very common.
So, discern the 'difference' between him having wakings DUE TO 'growth-spurts' & NEEDING to feed (which is common at 3-4 months old), and him just self-soothing and having normal hiccups in sleep/REM during the night...at which time, he's self-soothing through sucking and can go back to sleep.
All the best,
Susan