I'm new to Descent, how does line of sight affect range? A line of sight value of 4, a range roll of 6... does the line of sight counter the range roll? Also, a line of sight of 2 but no ranged weapon... does the hero still have to be adjacent to the minion or can they melee attack from 2 squares away (the line of sight value)?

Line of Sight and Range are two separate things, neither of them affect the other.

'Line of Sight' just means your ability to 'see' the enemy figure - they could be the other side of the map and as long as there's nothing in the way you would have line of sight to them.

The Range required for an attack is determined by counting spaces between you and the target space. The Range number rolled on the dice is the amount of range your attack achieves. This range result needs to equal or exceed the required range to the target or the attack will miss.

Attacking with a Melee weapon must target an adjacent space, the Range number rolled on the dice is irrelevant. If your Melee weapon has the Reach ability it can attack targets one additional square away, which is essentially range 2 but still no range is required or relevant on the dice.

Check out the CRRG for this sort of question, specifically the information on Attacks on p7, Stages of Attack on p6 and the appendix on Line of sight on p87.