A song can be four chords. That's not wrong, but if your chord progression is simple, then consider the Principles of contrast to make your song POP.
where do you put contrast in your song?
Where do you put contrast in your song? What are your song's elements?
Have you thought about how your song is laid out while the concept is fresh? I recommend as good practice to outline a few basic song-structures for the style of music you write. it could look like one of these examples:
Helps with efficiency to simply put things into a place where you dont have to worry about structure. Why use this approach? because it takes the struggle out of putting the songs together because that part is already done. Just build the pieces to fit and stick them in. Songwriting En-masse, just remember your Contrast considerations.
how can you switch it up without being abrasive?
So now that we're fitting the pieces in, I guess you'll need pieces first. This is where creativity and imagination come into play. During your daily (and i hope it's daily) practice, noodle around until you come up with something. Some folks like to play from scales or books for inspiration. others simply jam. The point is to come up with something that sounds good to you. Remember it's music. There isn't a one-size-fits-all approach to this stuff.
how can you switch it up without being abrasive
Abrupt transitions are not always bad. To me it depends on the characteristics of the genre, some being friendlier to rule-breaking than others.