Here is the actual San Diego Section CIF rule per the GreenBook:
Basically, it says that you can't "participate or practice" on a travel team (outside team) if HALF or MORE of the number of players required to start a contest is participating. So, you can play Over-the-line (3 players) (its referenced below) or have 4 players at a "practice".
600.1. IN THE CIFSDS DURING SEASON OF SPORT
a. School Team - The current-year San Diego Section Master Schedule, approved by the Board of Managers, sets the official commencement and conclusion of each sports season within the San Diego Section. During a particular season of sport, there are no CIFSDS restrictions on interscholastic practice or the association of interscholastic coaches and their athletes.
b. Outside Team - During a San Diego Section athlete’s season of sport, he or she may not participate or practice on an outside team in the same sport (Approved January 21, 2003 Board of Managers). For the purpose of this rule, an outside team is defined as a non-CIF team consisting of half or more of the number of players required to start a contest by the
respective National Federation rules book. Tackle football is considered to be a sport separate from touch or flag football. Examples of outside team sports that would be in conflict include (but not limited to) 3 on 3-basketball, slow pitch softball. Examples of outside team sports that would not be in conflict would be 2-person volleyball, 5 on 5-indoor soccer, and over-the-line softball.
As an only exception, athletes participating in the individual sports of swimming, diving, and gymnastics may practice during their season of sport, as unattached athletes, with club teams, which are established under the National Governing Board for that sport, so long as the athlete also practices with the school team, and the athlete is entered to compete as an
unattached athlete in the CIF-approved competitions at Bylaw 600. (Approved June 3, 2003 Board of Managers)
http://www.cifsds.org/pdf/GreenBook.pdf