How to partition a list in a specific way

I have a list:

a = {{1, 2, 3, 4, 1, 2, 3, 4}, {5, 6, 7, 8, 5, 6, 7, 8}};

Using Partition and Map I get:

Partition[#, 4]& /@ a

{{{1, 2, 3, 4}, {1, 2, 3, 4}}, {{5, 6, 7, 8}, {5, 6, 7, 8}}}

And now, I would like to get a list which looks like:

 {{{{1, 2},{3, 4}}, {{1, 2},{3, 4}}}, {{{5, 6},{7, 8}}, {{5, 6},{7, 8}}}}

I tried to play around with Partition and Map, but I did not manage to create such a list. Of course there are easy ways to do it using Do loops and AppendTo, but I very much need to avoid Do loops. Does anybody see the way to create the list?

Answer

There is a new function in Mathematica 9 - ArrayReshape:

ArrayReshape[a, {2, 2, 2, 2}]

{{{{1, 2}, {3, 4}}, {{1, 2}, {3, 4}}}, {{{5, 6}, {7, 8}}, {{5, 6}, {7, 8}}}}