If your hair doesn't like Nizoral (most people get pretty dry hair after using it) try something like Regenepure DR. That's what I'm using now. Nizoral did a great job getting rid of dandruff and stop the itch but now Regenepure DR is sufficient at not getting the dandruff back but it makes my hair a lot healthier than with Nizoral.
Also, that looks like a sizeable loss in 4 months yes. Maybe your body doesn't like finasteride, it is very possible. How much do you use? 1,25mg? 1mg? 0,5mg? 0,25mg? If you are at 1.x mg then try to reduce that amount if your shedding has gotten worse and worse for 4 months on finasteride.
Do you get any side effects from finasteride? How often do you use nizoral?