my 4k is still going strong after 3 years of abuse since i acquired it. was jap import with unknown kays on the clock. the engine was in good nick, i pulled the head and sump off, checked and retorqued the bearings. add some porting, skimming, cam and a few reliability adding mods.
it is a 20 year old engine, bottom end never rebuilt. still has the original rod bolts.
it hasn't had an easy life. has had plenty of work up around valve bounce (8 grand, still standard springs). have had quite a few people ask me if i've ever heard of a rev limiter at the motorkhanas ;)
unless you are planning on going over 8 grand i really don't think they are needed. a new set of orig bolts would be fine, cost dependent on toyota.
if you plan on going over 8 grand then yes, use them. then you really need to be lightening and balancing things, and start thinking about rod preparation. etc. $$$$$$
personally i think 2 grand plus spent on a 4k is better off spent on a 4ag conversion, unless you are building for a motorsport class. sorry jamie :)
it is not the size of the rod bolts that is most important. it is the tensile strength that matters, the grade of the rod bolt.
a 4k bottom end has more bearing journal surface area than an early 4ag.