You can have somewhere close to 20 men and at least 6 planes on J4.
The conventional wisdom would be to stay on your factory so more units are always together–stepping up and spreading out means he doesnt need a sea base to hit you.
Did Japan build the sea base on yunnan/hainan/fic J1-2? if he does he can reach India in one turn and the only square you can screen or block is SZ 37–otherwise he has 2 ways through.
Does he control Burma? if he cant land his planes on Burma only carrier capacity can come. This is why Japan may also want an airbase, but having both is a big investment.
If Japan is coming directly at India, that means a couple of things; the push on Russia, China, and Anzac will be weaker and the US doesn’t have to worry about the whole japan fleet for 2-3 turns. Japan may try to KIF (Kill India First) because it is the most strategic move–the factory lets him help Germany kill Russia and UK. This means less Japan money and the JDOW J1 focuses on getting all of UKPACs income, not killing it quickly, necessarily…
Id say usually go full turtle and max infantry. No china interventions, nothing fancy, just get a ton of stuff on india. Air can fly in from all over, moscow, africa, med, since moscow isnt on the ropes yet. He cant attack you on J2 if you leave the DD in SZ37. He can simply just move his whole fleet into or past yours, at peace, there is nothing you can do about it. Still, his planes can only reach so far and they’ll need a spot to land–Ceylon is the best choice then Burma and he should grab it. Once your UK fleet is dealt with, India is going down–everything can flee or you can have departed early, this is like the rebels on Hoth. Live to fight another day, the fighters head towards moscow. He can still only bring 6 ground units plus what walks through burma–and you can attack any of that with your UK air to help. 1 ship can block all his bombardments. In some cases you just leave a stack there to die (or die in west india) but that’s still important because it wastes a japan turn with transport use to kill it, a turn that otherwise would be spent securing the money or asia/africa. If Japan can be held off until J 4-5 (or fails entirely) then you can consider it a kind of stalemate, where you wasted Japan’s time. The decision of whether to make a stand or not all depends on whether Japan is set up to bring everything, its different game by game. If the USA is KJFing or bombing japan or taking over SZ 6 Japan cannot screw around they have to move every thing back the other direction (mustering at SZ 36) as soon as they can.
Its not so bad to build up a Red Sea UK navy with (2TT 1-2CV 1BB 1CA 1DD) all stuff you can build or scavenge up on UK1-4. Then, when this happens, if the UK has a bunch of stuff in the middle and near east, Japan has to keep fighting/chasing those guys and cant just turn around and obliterate russia or anzac.