If I want to buy new candy on the market, how large a sample should I
take to be within 0.04 based on a 90% confidence level.
will appreciate if someone can show me how this is done
regards
Jabeda

I'm not sure I completely understand your question, but there are a
number of sample size calculators on the internet.  Here are a few
links to try:

http://www.azplansite.com/samplesize.htm
http://www.grapentine.com/calculator.htm  
http://www.opinionsearch.com/en/tools/calculator.asp

Plugging in your numbers results in a sample size of 425.  (Depending
on the size of the population you're dealing with, you may need to add
the total population into the calculators.)

can you for your help I have managed to find a book in the library
which has explained to me thoroughly

it is solved with formula  n = p(1-p) [z/E]~2
n = 0.25 [1.65/0.04]~2
= 4.25
so the answer you stated is correct using the above procedure