>> (?<=a)(?<=a)(?<=a)(?<=a)

> There are four different points.
> If a1 before a2 and a2 before a3 and a3 before a4 and a4
> before something.

Sorry, that view doesn't make any sense.  A successful lookbehind assertion matches the empty string.  Same as the regexp

()()()()

matches 4 empty strings (and all the _same_ empty string) at any point.

> Otherwise repetition of assertion has no sense.

As I said before, it's "usually a silly thing to do".  It does make sense, just not _useful_ sense - it's "silly" ;-)

> If it has no sense, there should be an exception.

Why?  Code like

    i += 0

is usually pointless too, but it's not up to a programming language to force you to code only useful things.

It's easy to write to write regexps that are pointless.  For example, the regexp

(?=a)b

can never succeed.  Should that raise an exception?  Or should the regexp

(?=a)a

raise an exception because the (?=a) part is redundant?  Etc.