3.5 is a 12.% reduction and will work though up to 10% is the usual barometer. 4 cubes is optimal an unless you absolutely cannot make it work, the GP 4 cu box is best. Keep in mind that many (most) manufacturers usually recommend subwoofer boxes that are actually on the small side as it makes the fitment more desirable. When it comes to sub enclosures, bigger is usually better within reason. This is based on longstanding applied subwoofer enclosure design; It's physics and you cannot change that.