The jbj rimless 8 gal is a great setup .. I set one up for seahorses.. Ya, I'm selling it... lol didn't work out this time with the ponies. HMU if interested it's still running too. As far as 4 gallons.. Too hard to maintain temp I feel and parameters would be even harder. You can easily maintain an 8 a 3 gallon wc every 5 days or so ( dep on bioload of course) No problems , aside from cooling. You want to make sure it gets zero sun on it, and put near an interior wall. Here in IE I learned that first hand. Fun to keep a nano though
good luck!