One of my favorite workouts is a really good climbing strength endurance pump out.
One thing to note about this type of training, is that lactic acid is toxic for the muscles. Long term exposure to lactic acid can be detrimental to strength gains. Additionally, one does not need to do this type of training for long periods to see gains, and diminishing returns can onset quickly. So this type of training should be moderated. I'll usually do these workouts leading into the peak of a macrocycle, once a week, for four or five weeks in a row. If I haven't been training endurance, and I'm going to get on some ropes, I'll usually do this workout several days before the first day on.
What happens when you do the 4x4 workout, is your are forcing your muscles into a severe anaerobic energy consumption mode, producing lots of lactic acid. The lactic acid builds up in your muscles and you get pumped. In doing this, your muscles build up a tolerance to the lactic acid, and the lactic acid reservoir grows. This means you can produce more lactic acid, as your reservoir is larger, delaying muscle failure, and your muscles will still be able to operate while you have lots of lactic acid in them. Feeling the difference in your muscles from one week to the next is pretty incredible. You can make dramatic gains in a short period of time. Climbing through pump is very enjoyable, and if this workout is executed with high enough intensity, an endorphin buzz will result. MMMMM... so nice.
So how does the actual workout go?
Pick 4 boulder problems 2-4 grades below your max (your max is what you can send given a reasonable amount of time on the problem, maybe 10-20 tries).
These problems should not be tweaky or tenuous, you should know the problems, and be able to send each one consistently, even when tired.
Perform each problem back to back, all four in a row. Do not rest between problems - do not top out, do chalk up if you want to, but do jog to the next start and get back on the wall! Time how long it takes to do one circuit (generally its in the area of 2 minutes).
Rest for the amount of time it took you to do your circuit.
Repeat 3 times. (4 laps on your problems, thus the 4x4).
If you cannot complete your circuit, choose easier problems. You want to finish your circuit - barely. If you fall before the halfway point of a problem, get back on from where you fell, if you fall more than halfway through, move on to the next problem.
With this exercise you will get extremely pumped, I do not recommend doing this until the end of a workout, because its a completely debilitating exercise.
You also will want to choose problems that resemble the type of problems or routes you want to climb outdoors. For me I want to climb steep routes at the Red, so thats what my 4x4 training focuses on- steep boulder problems. If you have a particular route in mind that you are projecting, you can try to choose problems that will resemble the route. Like if your route has a crux about halfway, choose a V1, V3, V2, V1 for your four problems, then you have a crux roughly halfway through.
Stick to this for just a few weeks and you will see a dramatic increase in your strength endurance, that will no doubt help on long problems and routes.