characters are like letters, punctuation marks, spaces, numbers etc. in chinese it could also mean each separate character but you know ;) But don't confuse characters with words. Something with 40-140 characters is really little!
You're allowed 140 characters with or without a link.
4000 characters is approximately 500 words.
740
not very many...
about 140
3000 Characters is averaged between 2,000 words to 2,980 words.(allwordphone.com/count-words-characters.htm)
not very many...
740
Average is 2-3 words a second, so around 140 words.
That will depend on the words. However, in typing for measuring speed, a word is considered to be 5 characters.
Words are considered to be 3-7 characters average. However, normally the 500 characters would include the space between words. Assuming an average for this is 5 characters per word, you could probably get 80-100 words into 500 characters, depending how "big" the words are that you use.