I am not the most experienced or skilled player out there but I only play as Japanese and I have a PBEM where I am in late 42 like you and I have had some success against 4es. There is no real antidote for the 4e but there are some things you can do. I mass AAA at important bases and have as much radar and air support teams as possible. I use Ki45 KAI 1as for bases that are too far for fighters to escort. Well trained pilots in the early Ki45 can do very well against 4es. Set the Ki45 alt to the altitude the bombers are coming in at and you will have more success and curiously less losses (it seems like the best way for a defender to lose fighters to 4e defensive fire is to set your interceptors too high. Also, use your zeros (same rule for alt.), they can actually be pretty good against 4es if you have good pilots. The key is good pilots!! Remember that in defense you will lose fewer pilots and train them up fast while he will be losing planes and pilots. Also, if you can it is good to have well supplied and guarded bases with in decent LRCAP range, so that if he shuts down the airfield, you can get him with LRCAP on the next turns until you build back the airfield, this will also help you shuttle planes back a and forth.
Also, use the early Ki45 but not the others (b and c models) the 20mm and two 12.5mm seem to be better than the low accuracy 37mm.
Of course this is all against day bombing, after getting stung a few times he will go to night bombing where only the AAA will help because Japan has no night fighter until 44. Make sure your AAA teams have as many searchlights and radar as possible.
Also, I should say that I am playing the Bigbabes C mod and it seems that 4es may be a little easier than they were to deal with in other versions but then again, it may not, it is hard to say.
Hope this helps and I am sure you will get even better advice from the more experienced Japanese players out there.