I'm guessing that means it's consuming 4 micro amps, which, even with 15 shift registers, shouldn't cause an issue since the total load is a single LED plus the registers.
Depends on who's shift register you are using. For example, TI's is rated for 80uA. That's still a small amount of current. However, don't forget that whatever you are driving with those shift registers will increase your current draw.
I read a bit more on decoupling capacitors and it makes a lot more sense now. I don't think I'm going to try to put them into this project, since it's a prototype, unless it becomes apparent that they're necessary.
When you are spreading your power across that many ICs and components, decoupling capacitors are extremely important. Try and add caps anywhere you can to help your power distribution. Next time you build a prototype, keep in mind that generally a 0.1uF for each IC is generally a good idea. (And it is always easier to remove stuff later than to add it in.)