The 4-month sleep regression is the first real sleep regression in a baby's life, not to mention the regression you can really title a "regression" and not just the newborn learning curve. I remember that sleep regression vividly and somewhat painfully. I did not, by any means, deal with it gracefully (nor did my husband). And as it turns out, there are certain things you can only learn about your marriage after the 4-month sleep regression. Hey, you might as well learn them before you hit the 8-month sleep regression, the 11-month sleep regression and the 18-month sleep regression (and that's just before your kid turns 2). Ugh.
That first sleep regression is the most brutal of all of them. Most likely, you've just gotten your baby into some kind of sleep schedule and you might even be getting the most sleep you've been able to enjoy since your baby was born. Life is starting to feel downright manageable and then, bam. The 4-month sleep regression hits your house, and your marriage, like a ton of bricks. You finally start to see the end of the tunnel and all of a sudden someone shuts out the light.
I remember that 4-month sleep regression in a haze of panicked Googling. I also remember it as one of the most exhausting points in my marriage. My husband and I had started to work as a well-practiced team and someone moved the goal posts on us. It was rough, but we did learn more about our marriage during that (thankfully short) phase of our early parenting than at any point, before or after.