Are all beats created equal?
Not really. Some beats are more important than others. In fact, do yourself a favor, speak out the following rhythm: 1-2-3-4-1-2-3-4. Odds are you accented each 1 more than you did the other numbers. The natural strong accents in 4/4 time land on the first and third beat of each measure (with the first being more accented). We call these beats the strong beats, whereas beats 2 and 4 are the weak beats, giving you ONE, two, Three, four, ONE, two, Three, four.
Every time signature will have a different organization of strong and weak beats. In 3/4 time, for example, the first beat will be accented (strong) whereas beats two and three will be weak. But the one thing that all time signatures will have in common is that the first beat is always the strongest one. As such, that first beat is going to have a little more weight to it. We call this first beat the “downbeat.” Use these natural strong and weak beats to help guide your melodic and rhythmic choices.
What do you put in each bar?
Think of each measure as being an empty parking garage. What you store in it is 100% up to you. Say you have a four-car garage. You could park four cars there. Or maybe two limos. Or one long truck. Or maybe you could do two cars and eight bikes. Or maybe you leave some spots empty. It's really up to you how you fill the space up, as long as you don't go over. Meaning, if you only have four spots to fill, you can't put five cars in there. It's just not possible.
All of this translates identically to measures in music. You can fill each measure up with notes of any size/duration to your liking, as long as they all fit within the limits of that measure's duration. Meaning, if you're in 4/4 time, you have to fill four quarter notes worth of space in each measure. You could accomplish that with any of the following:
1 whole note
2 half notes
4 quarter notes
8 eighth notes
16 sixteenth notes
And these are only a few of the options you have. But if each measure was filled with exactly the same notes, that would get boring pretty quick. Well, you're in luck, because you can fill each measure with a mixture of any of the above as long as they equal exactly four quarter notes, for example:
1 quarter note, two eighth notes, one quarter note, and one quarter rest
3 eighth notes, two sixteenth notes, and one half note
Both of these add up to four full quarter note beats. And these are just two of a million options, so your choices are really limitless.
The important thing to remember is that once you add up all the notes and rests in each measure, they have to equal the total amount of beats defined by that time signature. Meaning, in 4/4 time, every measure has to contain no more and no less than exactly four quarter notes worth of music. Whether you fill that space with notes or rests is up to you.
Who uses 4/4 time?
Honestly, almost everyone in modern music chooses 4/4 time. It's a steady beat pattern that anyone can follow. From Lady Gaga to Beyoncé, to Adele, to your beloved indie band, odds are, the majority of your favorite songs are in four-four. Here are but a few of the millions of examples:
Mark Ronson's "Uptown Funk" ft. Bruno Mars
Carly Rae Jepsen's "Call Me Maybe"
Whitney Houston's "I Wanna Dance With Somebody"
Camila Cabello's "Havana" ft. Young Thug
If you open up any DAW (Logic Pro X, Pro Tools, Ableton Live...) your session will open up in 4/4 time by default so you can start writing in 4/4 right away. Most of the loops that are available on the market are also written in 4/4, so you have plenty to choose from when it comes to finding inspiration.