Cyst found on MRI, not seen on ultrasound or Mammogram

Just wanting to get a little advice (or assurance) on a lump I found about 15 months ago - April 2015 (@ 38 years old). Prior to finding the lump, I would have periodic pain that breast that I could only relate to the feeling of on oncoming clogged duct from breastfeeding (at that time I hadn't been breast feeding in over a year) I had my obgyn check it out. He said he thought it was tissue, and I could watch it or if I wanted to be extra cautious, get a mammo and u/s. I opted for the mammo and u/s. The mammo, didn't show anything, but said I had moderately to severely dense breasts. The ultrasound didn't show anything. The pain in that breast is not consistent. I tried cutting caffeine for about 2-3 months, but it didn't seem to change anything. I went to a Breast Surgeon, who didn't thought it felt like breast tissue and didn't recommend removing it unless it really starts to bother me. In October I had another ultrasound because I thought I could feel another lump, and it didn't show anything at all. The radiologist came in and said she thought it felt like tissue.

Because I was worried about a lump that nobody could see and periodic pain, my dr ordered an MRI. The MRI said there was a 4mm cyst. The radiologist said it was nothing of concern as did my doctor. I did seek a 2nd opinion from another Breast Surgeon who said it was nothing to be concerned about and to continue with yearly mammos/ultrasounds, and self checks.

I guess my biggest concern is if the MRI can tell for a fact that it's just a benign cyst and not cancerous. I think I tend to press the issue a little more as I know 2 women that are in their yearly 30's with breast cancer. They've never diagnosed me with anything, so I'm just wondering if this is common symptoms of a benign breast condition.

Thanks in advance!