1999 Toyota Harrier Failing to get to gear 4

I bought my ex Japanese car in 2011 December. Its a Toyota harrier, 4 wheel drive and is automatic. All along it was OK. The problem started in 2014 around August. It has difficulties in going to gear 4 from three. When I start it it can go to all gears up to 4 but if I lower the gears or stop it wont go up to gear 4. It will switch on the engine light and refuse. I have wasted money on the car but its not helpful. I changed the gear box but no improvement. I have also changed the air flow sensor but in vain. I took it for diagnosis when the engine light was on and it showed the error " Knock sensor in bank 2". Many mechanics have changed sensors too many a time but nothing have helped. I took it to one special mechanic last week and he said I should buy new ATF oil ( gear box oil ) and a new knock sensor. I haven't done so but my heart keeps telling me that nothing is going to improve.
what really could be be the problem?
I thank you very much for your cooperation and helpful answers.