Since this may be of interest to other people who come asking with the same questions, I finally located a reference to the 4" rule. It was in the 2006 International Residential Code.
http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_3_sec007.htm?bu2=undefined
SECTION R307 TOILET, BATH AND SHOWER SPACES
R307.1 Space required.
Fixtures shall be spaced as per Figure R307.1.
FIGURE R307.1 MINIMUM FIXTURE CLEARANCES
However, on the webpage that has the code it also has the 2009 and 2012 versions and the diagram showing the lavatories and the 4" distance was no longer included. When I looked for a section on lavatories, nothing was mentioned about distance from the wall.
The most recent code reference I could find online was the 2012 IRC.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_3_sec007.htm?bu2=undefined
SECTION R307 TOILET, BATH AND SHOWER SPACES
R307.1 Space required.
Fixtures shall be spaced in accordance with Figure R307.1, and in accordance with the requirements of Section P2705.1.
FIGURE R307.1 MINIMUM FIXTURE CLEARANCES
P2705.1 General.
The installation of fixtures shall conform to the following:
1. Floor-outlet or floor-mounted fixtures shall be secured to the drainage connection and to the floor, where so designed, by screws, bolts, washers, nuts and similar fasteners of copper, brass or other corrosion-resistant material.
2. Wall-hung fixtures shall be rigidly supported so that strain is not transmitted to the plumbing system.
3. Where fixtures come in contact with walls and floors, the contact area shall be water tight.
4. Plumbing fixtures shall be usable.
5. Water closets, lavatories and bidets. A water closet, lavatory or bidet shall not be set closer than 15 inches (381 mm) from its center to any side wall, partition or vanity or closer than 30 inches (762 mm) center-to-center between adjacent fixtures. There shall be a clearance of not less than 21 inches (533 mm) in front of a water closet, lavatory or bidet to any wall, fixture or door.
6. The location of piping, fixtures or equipment shall not interfere with the operation of windows or doors.
7. In flood hazard areas as established by Table R301.2(1), plumbing fixtures shall be located or installed in accordance with Section R322.1.7.
8. Integral fixture-fitting mounting surfaces on manufactured plumbing fixtures or plumbing fixtures constructed on site, shall meet the design requirements of ASME A112.19.2/CSA B45.1 or ASME A112.19.3/CSA B45.1.
There is no more mention of the 4" rule. Does this mean that it was eliminated?
If it is still in existence and I'm just not seeing it, do Euro-style vanities count as lavatories?
Edit: I found a document of proposed changes for 2007/2008 IRC and one of the suggestions was the removal of the 4" rule as it was impractical and not supported anywhere in the text of the code. So, I'm guessing that the proposal was accepted.
http://www.iccsafe.org/cs/codes/Documents/2007-08cycle/ProposedChanges/V2_RB1-100.pdf
I'm going to assume this means the 4" rule is not valid. I hope this is helpful to anyone who is wondering about this.