What does 4 days in 2 months mean? (or 5, or 6?)
A pass with 4 days in 2 months, means that you can choose any 4 days of travel, within a 2 month period. For example, your pass would be valid from 1 July until 31 August, and you could pick any 4 travel days during this period.
With a 5 day pass, you could pick 5 travel days etc.
A travelday on a Eurail Pass is the period from 12 midnight on one day until 12 midnight on the following day. You can travel as much as you like during that 24-hour period and hop on an off any train whenever you want (except for high speed trains which require seat reservations, they still count as one travel day, but you will need a reservation prior to boarding).
There is an extra bonus called the "7pm" rule that lets you travel after 7pm on a direct overnight train. You will only need to count the day of arrival as travel day (and fill it in on your Pass). You in effect could get an extra 5 hours for your day if you only use it after 7pm on a direct overnight train.