2 year to a 4 year school?

I'm studying Police Science currently at a 2 year school & I'm specializing in criminal behavior, but I want to finish getting my 2 year degree at my community college but then go to a 4 year & get a BD in Psychology. In order to do that would I have to do a 4 year term at the regular school or would I be able to finish in 2 years like my community college because I already have a degree with kind of deals with the same issues. Thank you for any advice!