It was probably about 15 years ago - we were mid-session in the bouldering gym and someone suggested we do some "4x4s." At first I thought this had something to do with the lumber of the same name, but in climbing, this is simply four sets of four boulder problems done back-to-back. The four sets are separated by a rest period of 3-4 minutes, usually about equal in time to the total time it takes to climb the four problems. We picked four problems, and took turns running through the sequence. The whole thing only took 25-30 minutes, but it destroyed us. I was sore for days.
Of course, soreness is a terrible indicator of a training session's effectiveness - about like judging a Mexican restaurant's meals by the volume of gas you produce afterward. What soreness really means is that you probably found a weakness in your training. Despite the fact that this weakness had reared its head, we probably went another year before we did the workout again.
Since that time, 4x4 workouts have become commonplace. As a way to increase a climber's anaerobic endurance, these short workouts are very effective. One caveat: they aren't worth a damn unless you repeat them regularly in the framework of a training plan.
There's nothing magic to the 4 boulder problem part. You'll see results with three or five in a row, too. The trick is making sure the problems are difficult enough to cause you to "redline," but not so hard that you blow your engine. Here are some steps to optimizing your training.
1. Plan it out. Don't get me wrong, doing 4x4s randomly throughout the year will beat the hell out of you. However, if you want to reap a reward from the pain you need to set up a plan to repeat the workout regularly over time. Most climbers will need 4-5 days between these hard sessions, some can get by on fewer. Usually, once a week for about 4-5 weeks is the best bang for your buck. Try to stick with the same 4 problems - this helps you see your progress, which helps you decide that it might be worth doing again.
2. Keep track. Write down things like the number of times you fell, how crappy the last set felt, and the rest times you took between sets. This information is really valuable to you if you're serious about training. Sure, your burned-out, slacklining, dope-smokin' "bros" will make fun of you. But you're trying to get better, right?
3. Stick to it. Go the whole way with this. The place you'll see the biggest improvement is when you're feeling "flat" toward the end of a cycle. If you can push through this time, you're in there. Keep your mind looking forward, and forget about feeling "fit" or "in shape," we're looking to surpass that level.
4. Make it specific. There's little that's more useless than cave-climbing fitness at Smith Rock. Think about this when you plan your sessions. Pick your problems so that they apply to the routes you want to climb. Hold type, wall angle, hell...even temperature. The better you train the better you perform.
The 4x4 is a great workout, but it's not the only tool in the box. Here are a few variations that might work better for you:
These sessions are done by repeating the same problem back-to-back with about five minutes' rest between. The problem you choose should be just at your onsight limit, and should "flow" well. You better enjoy it - you're going to do it 12 times. This is a great education in the value of repetition; your efficiency tends to improve as your physical resources are diminished.
We do a lot of these - they tend to be safer and more engaging for our athletes than the 4x4s...staying on the wall keeps them from chatting too much. These get noted as "N" efforts (up-down-up), "MI" (up-down-up-down-up), and "MN" efforts (up four problems and down three). Naturally, these need to be adjusted down in difficulty depending on the length of the set. Try to aim for 85% - an intensity that would allow for a bit more climbing at the end of each set. This keeps you from totally destroying yourself on the first one - and allows for more quality climbing per session. Getting pumped is not the goal, getting better at climbing is the goal.
Problems on the Clock
This workout is simply done by repeating efforts at regularly-spaced intervals. A normal effort might be 15 problems, doing one per minute for 15 minutes. Harder problems done, say, every three minutes for 45 minutes offer substantially more on the strength-endurance end. The shorter your rest period, the easier your problems must be.
To reiterate, the key to really increasing your climbing ability is to regularly repeat efforts, and slightly increase the difficulty. Just randomly doing boulder problems will, of course, lead to improvements. Getting there quickly, though, requires some planning. Do the homework.