Azumanga Daioh, K-On! Recommendations
The 4-panel format, character interaction, and setting is very similar in both manga, except Azumanga Daioh dosen't have a focous on music.
K-On and Azumanga Daioh have the same kind of pleasant charm. They're 4-koma series that revolve around a group of high school girls as they live out their lives. The main cast in both are lovable and have their quirks that make them stand out.
They're enjoyable and funny reads that will brighten your day.