A triple stop is the same technique applied over three strings, while a quadruple stop is over four strings (which is all the strings on a violin). Double, triple, and quadruple stopping are collectively known as multiple stopping.
On instruments having a curved bridge, it is difficult to bow more than two strings simultaneously. The style of bow used until around the end of the 18th century, particularly in Germany, had the wood curved outwards (away from the hair), which made it somewhat easier to play three notes at the same time. However, most treatises written around the time make it clear that composers did not expect three notes to be played at once, even though the notes may be written in a way as to suggest this. Playing four notes at once is almost impossible, even with older bows. The normal way of playing three or four note chords is to sound the lower notes briefly and allow them to ring while the bow plays the upper notes (a broken chord). This gives the illusion of a true triple or quadruple stop. In forte, however, even with a modern violin and bow it is quite possible to play three notes at once, especially when played a little more towards the fingerboard. Obviously, with this technique, a little more pressure than usual is needed on the bow, so this cannot be practised in softer passages. Of course, great skill is needed for the violinist to keep a beautiful sound. This technique is mainly used in music with great force, like Russian music (the most obvious of these is the cadenza-like solo at the beginning of the last movement of Tchaikovsky's violin concerto).
A 20th century invention by Emil Telmányi called the "Bach bow" uses a system of levers to temporarily slacken the bow hair and allow sustained three- or four-note chords; this design has no historical precedent, and is no more authentic than an ordinary modern bow for playing baroque music.
In addition to the style of bow, the curvature of the bridge is an important factor in the ease of multiple stopping. On most classical instruments, the bridge is curved enough to make it difficult to play three strings at once, but on some violins the bridge is shaved down until almost flat, making it far easier to triple stop, as well as to alternate double stopping on different pairs of strings (D-A to A-E for example). The compensating disadvantage is that more skill is needed to avoid playing a double stop when none is called for.
Multiple stops are also used in tuned percussion, such as on the vibraphone or marimba, and more rarely, timpani. A percussion double stop simply consists of striking both bars or timpani with two separate mallets.