Ventilation in shed

I have a question on ventilation.

The area that I have is 4' w x 4' h x 2' d box that I built in a shed without central heating/AC.

I have a small space heater for the colder temps now and a window AC unit that I will install when the summer comes.

How do you keep the ventilation up (fresh air) in the box without losing heat and using tons of electricity?

I don't want to heat/cool the whole shed either. <-- That's really the MAIN thing...

Thanks!
Mo

Only thing i could think of is using CO2 because by the sounds of it you dont want to be venting to often due to losing your temp range. You could have it so that your lights come on during the night to keep your night temps up and use CO2 than. and if the air isnt to cold during the day just have it exhaust normally during the day without the co2 on