Q:

Differences between 'till now', 'as of now','yet', 'up till now'

I have seen phrases like the following:

Till now
Yet
Up till now
As of now

Is there any difference between their usage and meaning, or do they have the same meaning?    

A:

Till has the same meaning until has; it is just informal, and not used at the beginning of a sentence.
"Till now" and "up until now" have the same meaning.  
Yet is used in negatives, and in those cases you could get it to mean that something didn't happen "up until now." Yet also mean "from now until a period of time mentioned."

They won't arrive for at least two hours yet.

I have never heard as off now being used. It could be "as of now" but I would understand that as "with regards to now." I heard "as of" used in sentences similar to "As of you, you would better be quick." and I am not sure "as of now" would be used with that meaning.
Oxford Dictionaries says that "as of" is used to indicate the time or date from which something starts.

As from 1 January, a free market will be created.

I'm on the dole as of now.

