Minimum number of colors I just read an old book today and it was stated that mathematicians are still unable to answer "What is the minimum number of colours needed to paint a map such that adjacent countries will not have the same colour" , so the mathematicians now know the answer? or is it still unknown !

The 4-color theorem has been proven. In my "Graphs and Digraphs" book by Chartrand and Lesniak (4ed 2005), the story is told that in 1890 Heawood proved the 5-color theorem as a result of spotting an error in a flawed 4-color theorem by Kempe a decade earlier. After 1890 we had the 5-color theorem, and the 4-color conjecture for many years. 
It was not until June 21, 1976 that the 4-color theorem was actually proved by Appel and Haken. Anyway, textbooks changed a little bit after then, but not much as the 5-color theorem is doable in about 1 textbook page, but the way in which Appel and Hanken proved the 4-color theorem was computer intensive and not conducive to inserting in a chapter on graph colorings.
This is the most likely explanation for your book. It was probably written before 1976. Note that I would not throw the book away or think of it as obsolete. We still cannot fit a proof of the 4-color theorem on one page of a textbook, although finding less computer dependent ways to prove 4-color has been a source of active research. Also note that the 5-color theorem proof is still a favorite of graph theory students due to its elegance and relative simplicity.
