4 month old male with only one testicle...should I give up or wait until 6 months?

Wait til 6 months.

If you think you will (or have already) get attached bail now. Every week that goes by lessens the odds and in my experience it's not likely to happen. Yes, the are all kinds of stories, but the odds are not in your favor.

Wait until he is 6-7 month old, which is when the testicles get bigger and naturally tend to "fall" in place.

If you can feel the one that still hasn't dropped but it's below the inguinal ring, you should wait.