what "regression" gets wrong
The word makes it sound like your baby went backwards and will bounce forward again if you wait. That is not what is happening. For the first three months, babies have a simple two-state kind of sleep. Around four months, that matures toward the adult cycle, with lighter and deeper stages and brief surfacings between them. That maturation is permanent. Your baby is not broken and you did not cause this. Their brain grew, on schedule.
the part nobody explains
Every person alive surfaces briefly between sleep cycles, adults included. You do it and do not remember, because you resettle so fast. Your four-month-old is doing the same thing for the first time, and they do not yet know how to get themselves back down. So the real question stops being "how do I stop the waking," which you cannot, and becomes "what does my baby find when they surface." If they fall asleep being fed or rocked at bedtime, that is the thing they will look for at 1am when they come up, and cannot recreate alone. That is why the fix, when there is one, usually lives at bedtime and not at the wake-up.
what actually helps
Two things do most of the work here. First, protect the timing, because an overtired four-month-old handles this transition far worse than a well-rested one, and timing runs on wake windows more than the clock. Second, give your baby low-stakes chances to practice settling without being fully put to sleep, gently, without turning bedtime into a standoff.
How far you take the second part depends entirely on your baby and on what you are comfortable with, and that is genuinely personal. It is the piece I build with each family rather than hand out, because the right version for a baby who has never slept alone is different from the right version for one who used to and stopped.
Anything about feeds or growth during this stretch, and babies do often feed more around now, is a question for your pediatrician, not for me.
quick answers
Should I ride out the 4-month regression or sleep train?
There is nothing to ride out. The change is permanent maturation, not a passing phase. The work is about what your baby finds when they surface between cycles, which is set at bedtime.
Is the 4-month sleep regression permanent?
The change in how your baby sleeps is permanent, because it is maturation. The rough patch while everyone adjusts is temporary. Confusing the two is why it feels like it will never end.
How long does the 4-month sleep regression last?
The adjustment is usually a couple of weeks. The new way of sleeping is forever, which sounds bleak and is not, because the mature cycle is also what eventually makes long consolidated sleep possible.
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