My Toyota 4 Runner (old model) misfires and seems lazy especially while climbing hills. There is also a delay in changing from gears 2 - 3 and 4. Can someone please help to advise? Thanks in advance.
I know this sounds crazy, but I think that you need to fix/replace the exhaust. Basically, in my humble guesstimation, you don't have enough compression in the exhaust system. Either you have a non-factory muffler or a hole in the pipe or your catalytic converter has been stolen, or something along those lines.
Toyota and Nissan (and probably a few others) behave like that when they don't have the right amount of banana in the tail-pipe (back pressure).
In my experience, when I replaced the exhaust system of a '92 Toyota truck and '92 Nissan truck (in the years of our lord 1998 and 2002, respectively) with cheap muffler replacements, I initially experienced an increase in power... which eventually subsided after a couple months and became the problem that you described.
Then, after replacing the one-size-fits-nothing muffler with factory exhaust parts, both vehicles had an inital loss of power, which eventually subsided and became normal operation.