go with the 4 gallon. nanos are harder than tanks over 10 gallons, and the smaller the more difficult it is. both in terms of nutrient dosing and CO2:light balance, and just the physical aspect of planting (hard to maneuver).
so you will go with high light and DIY CO2?
ok so first off you should make it easier on yourself by getting a good substrate. ADA aquasoil amazonia is the best substrate, so get some of that. otherwise you could mineralize some topsoil and cap it with fine grave or sand.
personally, i had some issue using a glass diffuser for DIY CO2, because it would only work after 3 days, and only for 5 days or so; whereas in a reactor it would work the day after setup and for 2 weeks. i believe it was a pressure issue, and that the yeast could keep the pressure high enough for most of the time to force the bubbles through.
you can shut it on and off very easily. you have several options: move the diffuser out of the tank, pull the tubing off, etc. you just have to shut it off manually.
but why shut it off? if your worried about your livestock just have the tank set up so that at night there is a lot of surface agitation (a powerhead or airpump on a timer will do nicely). but bettas are pretty CO2 tolerant.
you can never really measure CO2; not accurately anyway. the best way to tell is to observe you plants and your livestock. no pearling plants means too little, gasping fish means too much.
pressurized, is the way to go though. and its not as expensive as it seems, cuz its really just a startup cost. once you have the bottle, reg, solenoid, etc. thats it. you just have to fill it up yearly (or less often) for such a small tank, and thats not expensive at all. my advice, use DIY while you save up for pressurized.
no need to really balance nutrients. do EI dosing. read about it: http://www.barrreport.com/forumdispl...timative-Index