Hi ll
Or in other words, you are already DOING Cardiovascular training, you're running ;)
The gym will just give you other options, such as stationary bike, rower, eliptical (sp) etc, none of which should hurt your running.
I would caution however, that depending on your current level of fitness (You mention only running for 4 months) it is possible to overtrain by doing too much. This can make you feel tired and letargic (sp)and affect your running. Your body will still be getting use to your exercise regime, so if you do join a gym start slowly and as is always the case "Listen To Your Body".
Also don't forget you need to fuel your body for the extra work you will be asking it to do.
Good Luck
MK