"Щас" is a way to pronounce "сейчас" in informal speech. You will not see it in writing, except for places like web chats etc where people don't care about their spelling and like to make words look just the way they are pronounced in informal speech. "Сейчас" means now, at the present moment. It's also used as "Just a minute" . "Теперь" means "now", but implies a contrast to something in the past : "Раньше я не любила классическую музыку, а теперь люблю". "Пока" can mean "bye" (informal), or "while": "Пока дети смотрели телевизор, бабушка готовила ужин". It can also mean "so far/yet" : Я пока не хочу есть - I'm not hungry yet.

There is NO word "щас" in Russian!!! It's the fast pronounciation of "сейчас". Yes, sometimes "щас" is used in Internet slang, but it's incorrect and illiterate to write it in such a way.

щас -is informal. during speaking u can use it, but in writing it's a mistake. сейчас - means "now". теперь - i dont know how to say.. maybe "this time" or smth like that. пока- has several meanings. while is the most widespread I suppose.

Actually "щас" and "сейчас" means the same thing - 'right now'. Only "щас" is a shortened variant of "сейчас" used by common people in everyday life conversations just to sound faster. However, I would not recommend you to use "щас", especially in formal speach. It will not make any difference in use, but you will sound smarter saying "сейчас". About "теперь" and "пока" Olga gave pretty accurate information.

"щас" это сокращение от "сейчас" (щас=сейчас) "сейчас" и "теперь" в большинстве предложений будут иметь один и тот же смысл, но используя "сейчас" мы делаем акцент что это происходит именно в данный момент (в эту секунду) "пока" это аналог "while" в английском, не очень пойму почему это слово в одном списке с предидущими. Может быть Вы приведёте пример где оно у вас вызвает сомнения?