よつばと! / ǂc あずまきよひこ. (OCLC #71833438)
Most of our manga is classified under PN6790.J34; that is, PN6790 for comic books/strips by region or country (other countries), cuttered by .J3 for Japan. The table for this classification says that you can append 3 to further cutter by author, or append 4 to cutter by title, specifying that title is the preferred way.
This particular title (Yotsuba & !) has the call number:
PN6790.J34 Y68 2003
We also have some volumes of the English translation under:
PN6790.J34 Y6813 2005