I started with 4 inch computer fans. they did just OK.
Later I found that the refers are not normally installed properly. Big thing they are installed with too much distance between the cooling unit coils and the exterior wall. Instructions say less then an inch. Mine was over three inches. I closed space up with foam board.
Next looking at instructions, you need to block the void on the top of the refer. I used a peice of round duct to force the air out the upper vent. I had to remove the refer to accomplish this.
After fixing ventilation issues the refer has worked great.