If you look in your owner's manual, you'll see that the timing belt
replacement at 4 years/60k miles has a footnote which reads something like
"in California, replacing the timing belt is recommended but not required."
I suppose there's some California-specific legal issue which makes them
unable to force you to do it at your expense.
I can't provide much advice as to what you should do unless I know the
service history of the vehicle. Presuming your vehicle is 2001 or older,
here's my personal recommendations for maintenance and their intervals:
Replace trans fluid/filter -- 30k miles
Replace air filter -- 30k miles
Replace spark plugs -- 30k miles
Replace spark plug wires -- 60k miles
Replace fuel filter -- 60k miles
Rotate tires -- 7500 miles (depending also on tire condition)
Replace coolant -- every year
Replace timing belt -- 60k miles or 6 years
Change oil and filter -- 3k miles
Note that some items are a little different than the manual
recommendations. I've seen enough plug wire failures on the 2.0 engine
that I recommend replacing them at 60k. The Tiburon timing belt isn't one
that tends to fail frequently, so I wouldn't overly stress about the 4 year
deal. The fuel filter official recommendation is 52,500 miles, but I
figure that's close enough to just do it every 60k. And, given the price
of draining and refilling coolant, there's little reason to not do it
every year to keep everything hunky dory.
And, for others who might read this, these recommendations are intended
for '97-'01 Tiburon, '96-'00 Elantra, and '95-'05 Accent only.