A hexagon bolt, or hex bolt to give it its more common name, is a threaded bolt with a plain shank and hexagon shaped head.
Metric is the most common thread type in production today. Usually specified as M then a number x another number. This signifies the thread diameter and the length of the fastener.
A4 Stainless is often referred to as 316 or 18/10 stainless. As in A2 above, the numbers 18/10 refer to the chromium and nickel content- 18% chromium and 10% nickel. It is often used in marine or chemical environments as it has greater corrosion resistance than A2.
The chemical composition of both A2 and A4 stainless steel are almost identical with the most notable differences being the higher nickel content and the addition of Molybdenum in A4. Molybdenum is a silvery white metal that is highly corrosion resistant, very ductile and has an extremely high melting point.
There can also be Titanium present in A4 stainless at the option of the manufacturer.
Typically, A4 grade stainless steel costs slightly more than A2 because of its enhanced chemical and production properties. The extra cost can be justified, however, because hardware made of A4 stainless steel is extra resistant to tarnish and corrosion, enabling it to last a very long time.