Q: black screen and sleep light blinks 4 times

This is at boot?  Did you hear a boot tone?  Four flashes normally indicates that "no memory banks passed initial testing" which could be symptoms of something worse.

With which specific model of PowerBook are you having this problem?

The first thing to try is to reseat all RAM modules.

The second thing to try is to remove all but one module and try to boot, swapping other modules in and out until all have been tried. If you have only one module, try it in the other slot.  Finally, both RAM slots need to be tried with a single module, as occasionally one slot will fail. 

Eventually, though, if nothing works with the RAM you have in the machine, you need to try one known good RAM module in the machine to see if the machine will respond to either slot.

Final question: did you have any unexplained kernel panic events in the week or two leading up to this failure?