4 is incorrect, but the others are correct and used often.
4 would be correct if you followed it with a result -- It's been so long a time since I saw you last that I had almost forgotten what you look like!
1. It's been such a long time.
2. It's been a very long time.
3. It's been a while.
4. It's been so long a time.
===================
Are they all the same and grammatical?
Do you have other similar expressions?
2 answers
Its been so long to saw you even though I forget wat you look like
1 answer