Hi,

Your room is 4 ft wide X 5 ft long X 8 ft high = 160 cubic feet of space therefore the 160 cfm should be sufficient enough to change the air in your room every minute....
(as you might know,cfm= cubic feet minute)

If you think you might upsize you room in the future,then go for the 180 cfm that way you'll save extra expenses down the road but if you stick with this size room buy the 160 its more than enough and you save cash now.

Hope this helps.

QB