Minimum Line length for Line Distance Protection

The answer to the second question is "It depends" - on a number of factors, including available fault current, what type of relays you are planning to use (electromechanical or digital), what relay elements or characteristics you choose (mho impedance, with offset, lenticular, etc.), phase or memory voltage polarization capability of the relay, etc. I would think that at 4km long it would be possible, but I have not really done much in the area of line protection. A Zone 1 mho characteristic with offset might be what you need, since it protects behind the relay into the busbar. Or for that line length, if you can run fiber optics between the terminals (OPGW or fiber strung along the towers), you could do a line differential scheme instead.

For the first question, you might look at Areva's "Network Protection & Automation Guide", which is free to download when you request through their web site:

http://www.areva-td.com/solutions/US_930_US%3A%3ASolutions%3A%3AAutomation%3A%3ANPAG.html

Look particularly at chapter 11. And try to find someone who has experience with line distance protection to check and verify your work. A call to the relay manufacturer's applications engineer would probably get you some good assistance.