All rules must be execute. only thingh is the order..higher priority will execute first then lower …means that lower priority will overite the value.
as per your example
suppose input schema having a=1/b=1/c=1/d=1
Rule1(if a=1 then b=2) after execute
schema a=1/b=2/c=1/d=1
Rule 2(if c=3 then b=3) after execute
schema a=1/b=2/c=1/d=1
Rule 3(if d=4 then a=2) after execute
schema a=1/b=2/c=1/d=1
Rule 4(if a=2 then b=5) after execute
schema a=1/b=2/c=1/d=1
Rules are execute very simple ways or isolated way …pass xml file / BRE will check(if condision) the element value give the new value(action or set)/ pass or update the schema….
now we / developer have to think about ..priority on rule execusion…