SQL Express 2005 has a hard limit of 4GB. No way around that.
You can get SQL Express 2008 R2, which has a 10GB limit, but be aware that if you restore a database to that it will become a SQL 2008 R2 database and there will be no way to get it back to SQL 2005.
Otherwise you'll have to buy one of the licensed versions of SQL. If this is for development only (not a production server), then you can buy a copy of developer edition, which is very cheap.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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