How do you say happy fourth monthsary in Japanese?

I'm sorry, but I'm not even entirely sure what this means in English. Is it to be used in the context of "Happy [fourth month since we've met and been together] anniversary?" If so, I like it, and "4th month anniversary (of our dating)" would be 'tsukiatte yonkagetsu no kinenbi,' and can be written: 付き合って四ヶ月記念日

To say "happy" in the sense that we would in English, you can throw in 'omedetou,' written: おめでとう