To get 40 mg at once from 30 mg ER, I think taking four at the same time would be your best bet, assuming you want to swallow them whole. ER Adderall releases half of the total dose immediately (hitting about an hour after dosing, for me) and the other half about four hours later. Right? That's what I've always read, anyway. So if you take four 20 mgs at once, you'll get 40 mg about an hour later and another 40 mg about three hours after that, but the second 40 mg will be slightly weaker than usual, at least based on my experience, due to the lingering effects of the first 40 mg. When I've taken too many ER Adderall in the past, though, that four-hour bump has caused a lot of paranoia and negative side effects, especially uncomfortable cardiac ones, so be careful. Drink lots of water and be prepared to calm yourself down if need be.
If I were you, I'd crush the four 20 mg pills and parachute half of the powder after some Tums. Save the other half. This way, you get two 40 mg-level hits out of hour pills. If you take them whole all at once, you risk losing a good portion of the second 40 mg in the immediate comedown of your first hit. Whereas if you crush them and wait a few days between the two parachutes, you'll get two solid highs instead of one good high and one shitty shadow high. That's what I'd do, anyway. 40 mg is enough to fuck me up for several hours, but maybe I'm just a lightweight...