Ok, you know I think you're a spraylord

but let's set that aside for now, because I think this is a really interesting question that's been debated in different forms in this forum.

Here's one view: Lifting weights won't help.
The underlying assumption here is that specific training is necessary for gains that are going to be applicable to climbing. People that hold this view will argue that the joint angles and movements used in weight lifting aren't mechanically similar to movements in climbing and so you won't be helping yourself. In fact, by choosing to weightlift, you'll be taking time away from climbing and climbing-related recovery. If you buy this logic, then buy The Self Coached Climber and read the training plan at the end for "elite" climbers.

Here's another view: Weightlifting helps, you need to be stronger to climb better.
The underlying argument here is that many climbing moves are uber specific and climbing-training is too varied to adequately stimulate muscle growth so adding a weight lifting routine can be beneficial (see Performance Rockclimbing). Other arguments might include:
* Weightlifting will help increase the metabolism and increase muscle thereby increasing weightloss and/or improving strength to weight ratio.
* Weightlifing can improve weaknesses and complimentary muscles and thereby prevent weaknesses (e.g. tendonitis, shoulder injuries, etc.).
If you buy this logic, the check out Rockprodigy's training article and then find a weightlifting program for athletes (note that the set/rep scheme is different for athletes than bodybuilders) and integrate into your training schedule (you could start by searching on t-nation.com).

An alternative view: You don't know enough to make the right decision so learn more.
Okay, this might seem like I'm blasting you here, so try to read this with an open mind. As Bernard noted, 4 years isn't a long time. The fact that you 1) think you can't improve your technique much more and 2) don't know how to integrate a weight training program into your climbing program shows that you really need to understand training more.

First, the fact that you think you can't substantially improve your technique is a mental hurdle that you really need to blast through. Think about, one of the things that makes Chris Sharma such an amazing climber is that he's so humble. He never thinks he's learned all there is to know about how to move his body. He's always open to learning and because of that he tries things people would think are ridiculous.

Second, if you do go the weights route, there's a huge learning curve about how to train, eat, and rest that can't be distilled into a single post or thread. You're going to have to read a lot. You'll have to keep a training journal. You'll have to set specific goals. And you'll really have to monitor yourself so that you get the right amount of rest (the program you outlined at the start will simply lead to injury).

Finally, be honest with yourself about what it is you want. Dave Graham recently blogged about wanting to be stronger (and you can hear him psuedo-complaining about it in the recent Dosage vid). But is that what Dave really needs to break through to a higher level? Similarly, is that what you need or are you just climbing with guys with big biceps (like Dave climbing with Chris and Dani) and you think "if I had bigger biceps then I could climb that too." Honestly, I'm not sure what the answer is. Whatever you decide make sure that you're systematic and committed and that you are honest with yourself about why you made your decision.

Good luck.

...I also recommend Arna Ilgner's "The Rock Warrior's Way". This has helped many climbers break through to the "next" level, especially those who thought it was simply their physical abilities that were holding them back. Also, the book offers methods for learning how to recognize how to improve, and that is a powerful tool.

Correct me if I'm wrong seeing as I haven't read that book (although I've considered getting it), but isnt' "The Rock Warrior's Way" a guide to breaking through mental barriers instead of physical?

Yes. I think that is what I said, but I'll try to clarify the point I was trying to make. Removing mental barriers can enable more honest and effective assessments of what might be holding you back. Is it really how strong your lats are? Or is it your finger strength, your vision, your breathing, your core strength, your flexibility, etc.? I won't argue that it's not better to be stronger, but you might make bigger gains by focusing on what your real weaknesses are.

At any rate, you sound very much like me a few years back, and Ilgner's book helped me way more than the countless hours I spent trying to weight-lift my way to 5.12.

My $0.02.

Here's the question I should have asked before my long diatribe:

How do you structure your time in the climbing gym?

care to post the parameters of your routine?

lifts, sets, reps, days on/off?