submatrices into a bigger matrix

Hi

BY reading the docs I know how to split a large matrix into submatrices but how
do I do the reverse.  EG if I have 4 NxN matrices M1 M2 M3 and M4 - is there an
easy way to create a 2N x 2N matrix of the form

[
M1 M2
M3 M4
]

or do I have to just write a loop and do it element by element?  Thanks in
advance to anyone who replies to this.

Mike
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