You have to prove ties to home country, which is difficult to do when one says they are going to visit their romantic partner who just happens to be a US citizen.
They consider the strongest tie to be full-time employment. Which if you're taking 4 months off, I'm guessing you're not currently employed, correct? If you are employed, a letter from your employer stating you are on approved extended leave for whatever purpose (furlough, medical, seasonal lay-off etc...) and you are most definitely expected back to work on XX date is helpful.
Next would be a mortgage or lease/rental agreement for your permanent home. Mortgage more binding than rental of course.
Phone bills and utility bills, and bank accounts are secondary proof of ties, but easily enough abandoned that on their own are not quite convincing proof that you'll return.
Return ticket also is secondary and given that most return tickets cost less than one ways a lot of times, also easily enough abadoned and insufficient proof that you'll return. But helpful regardless.
I guess you're driving though, so I'd say just don't carry all of your worldly possessions in your car with you. Just bare bones, clothes, a few CDs, toiletries and sundries. No boxes loaded with books, kitchen utensils and rock concert posters. If you follow.
Never lie to a US CBP officer, they'll catch you in it sooner rather than later, that's their job.
Be truthful, but don't elaborate. When they ask you a question, give them short succinct answers, not your life story. Only answer what they ask, and look them in the eye, and be polite.
Extended stays can be tricky for anyone under the age of 55 mostly. But not entirely impossible. Even without ties it can be done, I did it myself a few years ago, but I think the CBP guy was hopped up on Halloween sugar and just in an especially good mood.

I had just been laid off from my job of 13 years, rec'd a substantial severance package, and sub-let my apartment to my Mum so I could spend the winter in NC with my husband, who at the time was awaiting Canadian Immigration decision. That last was the clincher, our intention was to live in Canada, so that was my binding tie.