A four-in-hand is a carriage

Carriage

A carriage is a wheeled vehicle for people, usually horse-drawn; litters and sedan chairs are excluded, since they are wheelless vehicles. The carriage is especially designed for private passenger use and for comfort or elegance, though some are also used to transport goods. It may be light,...

 drawn by a team of four horses having the rein

Rein

Reins are items of horse tack, used to direct a horse or other animal used for riding or driving. Reins can be made of leather, nylon, metal, or other materials, and attach to a bridle via either its bit or its noseband.-Use for riding:...

s rigged in such a way that it can be driven by a single driver. The stagecoach

Stagecoach

A stagecoach is a type of covered wagon for passengers and goods, strongly sprung and drawn by four horses, usually four-in-hand. Widely used before the introduction of railway transport, it made regular trips between stages or stations, which were places of rest provided for stagecoach travelers...

 and the tally-ho are usually four-in-hand coaches.

Before the four-in-hand rigging was developed, two drivers were needed to handle four horses. However, with a four-in-hand, the solo driver could handle all four horses by holding all the reins in one hand, thus the name.

The four-in-hand knot

Four-in-hand knot

The four-in-hand knot is a method of tying a necktie. Also known as a simple knot or schoolboy knot, the four-in-hand is believed to be the most popular method of tying ties due to its simplicity...

used to tie neckwear may have developed from a knot used in the rigging of the reins.