My client yesterday showed me her to-do list. Make that her to-do lists. The handwritten one on the yellow legal pad. The messages marked as unread on her Blackberry. The meeting action items listed on her iPad. The messages she flagged for followup in her Outlook inbox. Four lists, four places to look for work that needs her attention.
As I've written about before, 5S for the knowledge worker does not mean putting tape outlines around your stapler or setting rules about how many family photos can go on your desk. That's just a mindless transfer of 5S to the office. What you need to do is translate it for the office -- and that means applying it to the information you manage.
If you're living with four to-do lists, you need 5S. You need a way to organize the tasks so that you can easily see them, assess them, and make rapid judgments about what, how, and when to handle that work. If you're embarking on a scavenger hunt every time you want to plan your day, you're in trouble.
From my perspective, the twin purposes of information 5S are (1) to help surface abnormalities (in this case, work that's not getting done), and (2) to make it easier and faster to access materials. The fewer lists you have, the more likely it is that you'll accomplish those goals.
If, for some reason, you need to work with multiple to-do lists (the iPad for meeting notes, a legal pad for things you remember at your desk, the inbox for stuff arriving via email), that's okay -- but then it's incumbent upon you to 5S those lists each day: review them, consolidate items, and schedule the work in your calendar or on a personal kanban.
You wouldn't want to do something as simple as grocery shopping with four different shopping lists. Why would you want to do something as complex as scheduling and planning your work with four lists?