i use all kindsa planes, 3,4,5,7, block planes, etc. but for making small boxes, i don't see the point of a 4 or 5...they're bigger than a small box! now, if you're starting with rougher, larger pieces of lumber that you plan to cut up into smaller pieces, that's different...but for small pieces of wood, i can't imagine using a 4 or a 5...
but if you're bent on getting one, the #4 is the "all-around general best choice" plane...#5 is bigger, used for rougher work...#4 is smaller, used mostly for smoothing...
may i suggest searching ebay for an antique stanley? many of these are as good or better than today's record planes and you can find a greater variety of sizes...the records and stanleys are the exact same design..basically interchangeable...#3s are fairly common and can be had at a good price...and they're smaller than a #4...#2s are more rare...#1, which might be perfect for boxes, forget it...very rare and very expensive...
just make sure you look for a good useable plane...it's kinda trendy to collect these things nowadays so lots of fellers are trying to sell better condition stanleys at high prices...you don't need that, unless you're planning to hang it over the fireplace and look at it all day....at least half my planes are antique stanleys gotten off ebay...i don't think i paid more than $40 for #3 or a #4....but that was a few years ago...
