Riddle solver?

What is a word made up of 4 letters yet is also made up of 3. Although is written with 8 letters, and then with 4. Rarely consists of 6, and is never written with 5.

One example of where you will find me is many locations inside of one building. When I'm new I'm plain but after first use I'm never the same people clean me a lot but I'm never like new the changes come off but leave residue. You will have seen me in your early years I'm definantly a tool for the...

A river surrounds me dark nights inside me I keep outsiders out and insiders in what am I