Sounds like you are doing enough motions for triceps. So, go ahead and increase the weight. See what 8 lbs. does.

As for your knees, the pain you are feeling suggests that you are starining your patella tendon. Your form sounds good, but maybe you are going too deep? Try doing partials (not going down as far) and see if the pain begins to subside.

If it doesn't after 2 weeks, stop doing the lunges and see what happens.