Personally speaking, i always say go for the biggest you can maintain and afford...
Since as most of us know, it wont be long till you want to go bigger
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I have a 75 and 90 and trust me my fish love the extra height, they constantly swim all around including the top. 15 gallons of water is a decent difference but I agree for a 4' that a 120g is ideal. In a few months you'll look back and wish u got something even bigger! If your options are 75 or 90 go with the 90!
#1 S. Vettel