Longest Column in multi-column Excel Range (in a single formula)?

In a range where different columns may have different numbers of values (but there won't be gaps in the sequence within a column) I'd like to know the length of the longest column. To be more demanding, I'd like it in a single formula.

So given a range like this:

      |  A   B   C   D   ---+---+---+---+---    1 |  1   2   3   4    2 |  2   9   5   6    3 |      5   5    4 |          4    5 |

.. the answer I want is 4.

Something that works in this case is

=MAX(COUNT(A1:A999),COUNT(B1:B999),COUNT(C1:C999),Count(D1:D999))

But it's (a) ugly and (b) inflexible - doesn't cater for the case where a column might be added.

I tried something like this, hoping for an array giving the set of column counts (hoping to wrap a MAX() around it):

{=COUNT(OFFSET(A1,0,COLUMN(A1:D1)-COLUMN(A1),999,1))}

.. but it only gives a single cell, which always seems to be the count on the first column.

I know this could be easily solved in VBA but for several reasons (not least curiosity) I'd like to see if it can be solved in a single (probably array) formula.