Leaving the old MySQL 4 out of the equation, any of these databases will do a fine job serving as the backend for a small website. Given the choice, however, I would go for PostgreSQL.
Compared to MS SQL Server, it's an open source database which – all things equal – I believe actually improves your odds of getting through to someone familiar with the inner workings of the system. Compared to MySQL 5, it's a more mature product, especially in the area of transaction support. (As an example: in MySQL, DDL statements such as ALTER TABLE cannot be guarded by transactions).
PostgreSQL has a reputation for being a hackers database, but it's solely based on its extensibility. The database user is given a lot of power, including the ability to define custom types and perform advanced rewrites of queries. None of this will get in your way as a inexperienced user. But it's there in case you need it.
If you do decide to go for MySQL 5, be sure to stay clear of the default MyISAM table as it offers no transaction or foreign key support. InnoDB is what you'll need.