What is the difference between four stroke engine and six stroke engine?

The basic difference is there are two 'power' strokes in the six stroke cycle.

The extra power stroke is obtained by utilising what would be waste thermal energy to heat air (or steam) and injecting this into the cylinder at the appropriate time to provide the extra power stroke.

There are claims of up to a 40% reduction in fuel consumptions and reduced emission.

However, there are addition complications with the extra parts, timing etc., required and subsequent increase in weight.

There are no commercially viable ones on the market generally at present though with future improvements this may change.