40 width is right, 14/4, with logistics. And tons and tons of 20w marines. Keeping up with Allies is largely done thanks to focus - mass produce zeros with the focus/decisions, etc. However, you won't be able to keep up with the US on naval doctrines - they'll likely have most/all theirs done by the time they join the party, so it's a case of loss mitigation. Your only counter is kamikaze, which will keep their fleets away from your main islands.
Your key task really as Japan in MP is two-fold:
1) Take Malaysia and the Dutch East Indies to give them a huge headache with rubber. You really want to do it before the US is allowed to join, otherwise they can defend it all quite easily. That's the hard part, doing it quickly enough. If however you do take all the rubber, you'll get their attention, I assure you.
2) Your second role is to provide a distraction/knock out the RAJ (and their considerable manpower)/use your fleet to stop the Allies having a complete walk in the park at sea.
However, let's be honest, the Pacific theater is a backwater, so a lot of Japan players do get frustrated because they're kind of ignored. I'm afraid that, as far as the allies are concerned, that's both logical and historically accurate.
PS: many games don't allow you to bypass the Philippines so taking out Malaysia/East Indies early will not be an option. Then really you do have a challenge to take out the rubber - the US will just cover Malaysia with 40w infantry towers, and the RAJ may help too. Taking them out is going to be tricky, but that's the fun part of the game.