noun
B4 paper size
Refers specifically to the B4 paper size (257 mm × 364 mm), part of the JIS B series.
このプリンターはＢ４判まで印刷できます。
This printer can print up to B4 size.
Ｂ４判の用紙を買ってきてください。
Please buy some B4 size paper.
A4 is the more common office paper size (210 mm × 297 mm); B4 is larger and often used for newspapers, posters, or larger documents.
B5 is the next smaller size in the B series (182 mm × 257 mm), commonly used for notebooks and textbooks.
From the JIS B series paper size designation, where 'B' indicates the series and '4' the size number.