4-wide

From Openwaterpedia

noun - 4-wide (Four-wide) is a formation of four open water swimmers or triathletes swimming side-by-side during a race or training session.

There was a 4-wide coming around the last turn buoy.

Its equivalents (3-wide, 5-wide, 6-wide, etc.) are used when there are more individuals swimming side-by-side.

[Origin: modern-day NASCAR term].

4-wide example

with Darren Miller, Jamie Patrick, Jen Schumacher and Oliver Wilkinson across the Strait of Gibraltar