Database size limit of 4 GB in sql express 2005 only applies to Data file (.mdf) not for log files (.ldf)
As per MSDN
"The 4 GB database size limit applies only to data files and not to log files."
please visit this link
http://msdn.microsoft.com/en-us/library/ms345154(SQL.90).aspx
So you can have log file of any size but be careful with data file since its limit is 4GB and your data file will not be allowed to grow more than that even if you have mutiple data files for a single database.
Thanks,
Amit Kulkarni