Why does hydrogen spectrum contains 4 coloured lines why not 3 or 7?

In theory, hydrogen has infinitely many spectral lines. The "Balmer series" has four lines in the visible spectrum; additional lines are in the ultraviolet. Other "series" have other lines - it seems that all of them are either in the ultraviolet or infrared. For more information, read the Wikipedia article on "Hydrogen spectrum". The reason there are four VISIBLE lines is basically chance - an excited hydrogen atom emits light at certain frequencies (which can be calculated, see the article for more details); our eyes see a certain range of electromagnetic waves that happens to include four of those lines.