Hi, Ewa.
All these words are built of the three roots (ka ta ba), which has the meaning of "writing".
"kitabun" means something written (a book). Grammatically it's a noun, singular, nominative, indefinite.
"al - kitabu" has same meaning. (the book). Grammatically it's a noun, singular, nominative, definite.
"katabtu" is the verb (to write) conjugated to the first person, singular, past tens. (I wrote)
"katibun" is the active Participle form built of the verb and means (the one, who is doing the action) = writer .
Grammatically it's a noun, singular, nominative, indefinite.