4 Months Old: Development, Milestones, and the Famous Sleep Regression

Four months is a month of significant change. Your baby is doing things that are unmistakably intentional — reaching, rolling, laughing — and they're more interested in the world around them than ever before. It's a brilliant age. It can also be an exhausting one, because four months is when sleep architecture permanently changes, and the impact can be significant.

Motor Development at Four Months

Rolling

Rolling is one of the headline milestones of this month. Most babies roll from tummy to back first, as the arms can help push them over. Rolling back to front typically follows within a few weeks. Some babies won't roll until five or six months, which is still within the normal range.

Once rolling starts, the rules about putting babies to sleep on their back still apply — you place them on their back, and if they roll, that's fine. But flat surfaces with clear space all around them become even more important. No bouncy chairs, sofas, or cushioned areas for unsupervised time.

Reaching Accurately

At two to three months, your baby was swiping in the general direction of things. By four months, the reaching is becoming more accurate. They can bat at a hanging toy and make contact, grab your finger intentionally, and reach toward your face. Hand-eye coordination is developing rapidly.

Tummy Time Progress

By four months, many babies can push up onto extended arms during tummy time, lifting their chest off the floor. Some will start to pivot — rotating their body in a circle while on their tummy. This is a precursor to crawling.

Social and Communication Development

Laughing Out Loud

If you didn't get a first laugh at three months, you'll almost certainly get it at four. By now, laughter is becoming easier to trigger — raspberries, funny sounds, bouncing games, tickling. Your baby has opinions about what's funny and will make them known.

Babbling Begins

Beyond cooing, you may start to hear early consonant sounds — "bah," "mah," "gah." This is the beginning of babbling. It's still a long way from words, but the machinery is running.

Interest in the World

Your baby is now fascinated by their environment. They stare at faces, lights, patterns, and movement. They may try to grab anything within reach, including your food, your phone, and your hair. This curiosity is healthy and should be encouraged.

The Four-Month Sleep Regression

This deserves its own section because it catches so many families off guard.

Around four months, sleep architecture permanently changes. Newborn sleep is relatively deep and they cycle through it differently to older babies. From around four months, babies begin to cycle through sleep stages the way adults do — lighter stages, brief partial wakings, deeper stages — on roughly 45-minute cycles.

The problem is that if your baby has been going to sleep one way (fed to sleep, rocked to sleep, held to sleep), they will expect that same condition every time they surface into a light sleep stage. Which is every 45 minutes or so. A baby who was sleeping five or six hours suddenly starts waking every 90 minutes or every hour.

Why It's Permanent

The four-month regression isn't a phase that passes and leaves you with the same sleep you had before. The change in sleep architecture is permanent. What changes is your baby's ability to manage that transition between sleep cycles independently — and that's something that can be worked on over time.

What You Can Do

• Introduce a bedtime routine if you don't have one — bath, feed, song, dark room, put down. Consistency helps
• Try putting your baby down drowsy but awake — not fully asleep — so they practice falling asleep without needing you to recreate the exact conditions
• Check awake windows — at four months, awake windows are around 90–120 minutes. Overtiredness makes sleep worse
• Formal sleep training is generally not recommended before six months, but laying these foundations now helps

Feeding at Four Months

Feeding is usually well established at four months. You may notice your baby becoming distractible during feeds — pulling off and looking around at any noise or movement. A calmer feeding environment can help.

Do not start solids at four months. The NHS recommendation is to wait until six months. The gut is not ready, and early introduction is associated with increased allergy risk. Some older guidance suggested four months was an option; current NHS advice is six months.

What to Mention to Your Health Visitor

Speak to your health visitor or GP if:

• Your baby is not laughing or smiling by four months
• There is no reaching or swiping at objects
• Your baby doesn't turn toward sounds
• Head control is still very poor
• Movements seem asymmetrical — one side consistently more active or controlled than the other
• You're concerned about weight gain or feeding

Tummy Time is Still Important

Some parents ease off tummy time once their baby can roll, but it remains valuable for building the strength needed for crawling. Aim for short sessions spread throughout the day rather than one long stretch.