Have one fibroid, 4cm, and gyn wants tooperate. Why should i?

Depends. It depends on its location and if you have previously miscarried...Many women have fibroids and may not have any impact on the pregnancy. Generally if larger than 5cm- they have grown during a pregnancy- you miscarried.. Then one would consider removing.
Your choice. If you are having abnormal bleeding, pain, urinary frequency from the pressure of the fibroid, or you have been unable to get pregnant with the fibroid in place consider removal. Ask appropriate questions of your md as their may be other reasons that he/she may have for removal.