One trait that ALL great musicians share is that they’ve developed this ability to naturally feel 4-Bar and 8-Bar phrases. Here’s the low down:
WHAT’s a bar phrase?
A bar phrase is like, “ok, a guy walks into a bar…” nah just kidding…
Music is measured in what are called measures or bars (two different words for the same thing) which simply mean:
a small segment of music that holds a certain number of beats.
The time signature tells us how many beats are in each bar, and the VAST MAJORITY of popular music has 4 beats per bar.
For today, we’ll consider a bar to be a segment of music lasting 4 beats.
WHY is it important?
90% of popular songs are structured in 4 and 8-bar phrases. The 12 bar blues is a great example (made up of three 4-bar phrases) but nearly ALL popular songs are some multiple of 4 bars, so developing the ability to feel them naturally will level up your musicianship, and make you more popular with other musicians. Here’s why…
Ready for Action
As any racer can tell you, it’s never fun to be late off the starting block.
I’ve been caught off guard by leaders pointing at me to take a solo before I’m ready. Oops! My harmonica’s upside down, I fumbling to get my bearings, and by the time I’m getting into it, the time to solo is over and the band is on to the next thing! Doh!
Once you’re feeling 4-bar and 8-bar phrases, you can easily anticipate when the leader may cue you to start your solo, before they even give you the cue, so you can get on the good foot right from the start.
Communication is Key
When you can feel 4-bar phrases, you know when to make eye contact with the leader to confirm if you should be finishing your solo or going another round.
Tell a Compelling Story
The best solos feel like they’re telling a story by clearly having a Beginning, a Middle, and an End. How could you do that if you don’t even know where the end is? Developing the ability to feel 4-bar and 8-bar phrases will increase your awareness of context, and CONTEXT IS EVERYTHING.
We all get lost knowing where we are within the form of a song from time to time, but when we can REALLY FEEL these 4 and 8-bar phrases, we can easily find our place and quickly get back on track.
HOW do you develop this ability?
I’m glad you asked. 
I spent a period of months and years working on this, and it has paid rich dividends for me. Time well spent!
Learn to count
The way to count bars is to count to 4 in time with the music, and after the first bar, replace the number one with the bar number going like this:
ONE,2,3,4, TWO,2,3,4, THREE,2,3,4, FOUR,2,3,4….
After you’ve counted 4 bars, reset your counting to ONE,2,3,4
Use your fingers to keep track of how many 4 bar phrases you’ve counted.
Develop the habit of listening actively
Listening to music passively means simply enjoying it, or having it on in the background while you do something else.
But WHILE you enjoy it or do something else, you can also pay close attention to the details and really STUDY what’s going on. This is what we mean by ACTIVE LISTENING.
Active listening is as important to your musical growth as is actually practicing the harmonica. And as with anything, the more you do it, the more it becomes a habit.
Counting is one way to listen actively, paying attention to how the bars create the form: “Oh, okay it’s a 4 bar intro, a 16 bar verse, a 16 bar chorus, and then a 4 bar instrumental”
The more you practice this, the more you’ll NATURALLY start to FEEL these phrases, and they literally appear everywhere in music across genres.
You could think of counting as training wheels. The purpose of counting music is eventually to dispense with the counting and just to FEEL it.