Okay I'm on a four times a week workout plan.

On the day that I work back/biceps, it involved Assisted (wide grip) Pullups, Seated Rowing, lateral narrow grip pull down, dumbbell hammer curls, and a standard bicep curling machine.

I did this all summer, since may really, and I haven't really noticed a strength increase in my biceps. I've noticed increases in other areas of my body so it is just a little frustrating that I've seen no results here.

Should I give different exercises a try? Does anyone have any suggestions? It would be appreciated.

Those exercises are hitting your biceps well. Be sure you are varying the set/rep scheme every week or so.

3x10-12
4x6-8
5x5
for example.

Could be your diet. To build muscle you need to be in Bulking mode. In other words surplus in cals.

Stuff the bicep curls.

For biceps, I'd highly recommend chinups (inwards grip), as the best exercise hands down.
http://www.beastskills.com/chin%20grip.jpg
But probably not as close as in that picture.

Chinups are different to pullups - but both are important. What I do is swap them around at every workout. I would recommend you do the same, and see what happens.

starcraftmazter whats you MT line?

What is your diet like and what other compound movements are you performing?

No Idea, sorry.

go find it, search out chillens ubiqituous post, google it, read a book.....

What is your diet?? It could be that but I would include the chinups in your bicep technique. That is by far the best stimulation technique for the biceps. Include that technique everytime you work on your biceps and do it consistanly on top of adding lots of calories, high protein/carb dieting and you'll be fine. I would give it a 3 month goal.