Search for lines that do not have certain characters in a row

Question

I have a news article which I need to edit up a bit. Basically, I need to search for all lines that do not contain characters t and h 2 or more times in a row and replace 4 letter words with character D in those lines. Have no idea where to start. For example, I have this text

Animals are multicellular eukaryotic organisms which form a biological kingdom Animalia. 
Historically, Aristotle divided animals into those with blood and those without

And I want it to look like

Animals are multicellular eukaryotic organisms which D a biological kingdom Animalia.
Historically, Aristotle divided animals into those with blood and those without

Answer 1

Not too hard, really, as such tasks can be translated into regexps in a very straightforward manner.

:v/tt\|hh/s/\<\a\{4}\>/D/g

:v/.../ for all lines NOT matching a pattern

tt\|hh "tt" OR "hh"

s/.../D/g substitute ALL matches on a line with "D"

\<...\> "word" borders

\a\{4} exactly four Latin letters

Have no idea where to start.

Read about regexps. In Vim's embedded help these are :h usr_27.txt and :h pattern.txt.