Letter and A4 are close in area and not the same shape. Letter is 8.5 by 11 inches, the common U.S. copy paper. A4 is 210 by 297 millimeters, the common size in much of the rest of the world. A Letter sheet is a little wider and a little shorter than A4.
This site defaults to Letter because the domain and the language of the pages are for a U.S. reader. The A4 option is there for a tray that is loaded with A4, or for someone printing while traveling. The generator does not convert a Letter PDF after the fact; it draws the grid for the size you selected.
If you print a Letter layout on A4 paper with “fit to page,” the calendar shrinks and the margins look uneven. If you print an A4 layout on Letter paper the same way, the right or bottom edge can clip. Match the control to the paper. Do not ask the printer to translate sizes.
Orientation still matters. Letter landscape is 11 by 8.5 inches. A4 landscape is 297 by 210 millimeters. A weekly hourly grid needs that long side. A classic wall month usually stays portrait.
Half-letter (5.5 by 8.5 inches) is not a separate printer size on most home machines. The compact two-up preset stacks two small months on one Letter sheet instead, which is the practical substitute.
Card stock and ordinary copy paper both work. Heavier stock helps a wall month sit flatter. The template does not require color. Ink-saver gray is enough for an office copier.