I've found every small dehumidifier, even the ones that are $80, are completely garbage.
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I have 1 of these in each 4x4 and 1 works in a 4x8 no problem. They are cheap and relatively small. On the "low" setting they run for about 10 hours before you have to empty it. Mine also pull just about 100 watts on the low setting. They bring my tent humidity down about 15% during the day and 20% at night.
Also very helpful is to run your 6in exhaust at almost the highest setting. Pulling air out of the tent is how you get moisture out. It's essential if you can vent the air out of the window, into an attic, or as far away from your grow space as possible. This takes about about 12%-18% humdty for me. Without a dehu and proper venting I run around 75-85% Rh. With both of those my tent are 40-55%.