Why it is four sheets and not two
On paper A3 is two A4 sheets side by side. In practice every printer keeps an unprintable margin of several millimetres all round, and joined sheets need a little overlap to glue. Once both are accounted for, two sheets no longer reach across A3, so the layout becomes a 2 × 2 grid of four.
You can get closer to two by reducing the printer margin in the settings — but only as far as your printer actually reaches, or the edges of your poster will be clipped.
Getting the sheets to line up
The single most common cause of misaligned sheets is the printer rescaling the PDF. In the print dialog set the scale to 100% and turn off "Fit to page" or "Shrink oversized pages". Nothing else matters as much.
Print one sheet first and measure it against the size shown on screen. If it is short, the printer is still scaling.
Cutting and joining
Each sheet is printed with a solid line to cut along and a dashed line showing where the next sheet lands. Trim the white margin off the right and bottom edge of each sheet only — the outer edges of the poster stay untouched.
Lay the sheets out using the printed numbers, then tape from the back. The assembly map on page one shows the whole grid if you lose track.
Going bigger than A3
The same PDF works for any size. A2 is about eight sheets, A1 about fifteen and A0 about twenty-five, and you can also type an exact measurement in centimetres, inches or feet instead of picking a paper name.
The paper-use meter shows when a poster stops just past a sheet boundary and is paying for a whole extra row — nudging the size down slightly often saves several sheets for no visible difference.