How can I memorise easily the four tones?
Words do not generally have tones. Characters have tones, and they are integral to the character.

Memorizing batches of characters with certain tones is like memorizing batches of characters with certain pronunciations, that is, rather pointless.

When memorizing a character, you memorize its strokes and components, its pinyin transliterations, its tones (often more than one), and a number of common words formed by it.