Note that you're getting a fairly dumbed-down view of the whole virus issue so far. People like to play word games with this topic. It's important to understand that malware does exist for the Mac, so that you don't let a false sense of security behave in dangerous ways. Of course, there's been several orders of magnitude less malware released for the Mac in the last 10 years than appears for Windows every day, on average, so you'll probably never see any malware... but it's still possible. For the full story on Mac malware, see my Mac Malware Guide. (Note that my pages contain links to other pages that promote my services, and this should not be taken as an endorsement of my services by Apple.)
Also, regarding whether the stock 13" MBP has enough memory for you, it is impossible to answer accurately without knowing what you plan on doing with your computer. If you're a graphic designer, for example, who needs a lot of memory and power, 4 GB may not be enough, and for that matter, a 13" MBP might not be right for you. What do you plan on doing with it?