I am trying to find out if it is Legal for a company to retain an employee for four years and the only pay increase she has recieved was when Min. Wage increased. My Girlfriend hasnt had a raise in four years.

There is no law requiring increases in pay aside from requiring that the employer comply with minimum wage laws.

That aside, if your girlfriend has been working for minimum wage for four years, there is a problem. Either she is not worth more (and any increase in the minimum wage does not make her more valuable) or the company does not value its long term employees.

Barring a contract or CBA that specifically says otherwise, the ONLY time an employee is due a raise under the law is if they are working under the new minimum when minimum wage is raised by the appropriate legislative body.