1994 Nissan Pathfinder Gears grinding a little when shiftin

I recently borrowed my sons car to drive for a month or two. As I was driving it home I heard the gears grinding a little when you shift into 4th gear. He said that it just started. I also noticed that there is a whining noise when the car is in reverse and the clutch is in. Please help, I don't know if this is an indication that the clutch is going out or just needs to be adjusted. I was hoping that you could help me with suggestions about the possible cause of the problem. If it is the clutch, does it need to be fixed quickly before the "grinding" hurts the gears? Could I continue to drive it (for a short time) but avoid 4th gear? Last, if I make an appointment for repair, would you guess it to be for a clutch adjustment, clutch replacement, or is a whole new transmission involved. Yikes!