[R] more classes to one class in one dataset

[Jan Sabee]

I have a big database which more classes in class variable. I want to
make each class to one dataset, for example:
x1 x2 x3 x4  class
a  b  a  c  c    M1
c  b  b  c  c    M4
c  c  a  c  c    M2
c  a  c  a  a    M2
c  c  a  a  a    M1
c  a  b  c  a    M3
c  c  a  b  c    M3
c  a  c  a  b    M2
c  c  a  b  a    M1

How can I make, like:

x1 x2 x3 x4  class
a  b  a  c  c    M1
c  c  a  a  a    M1
c  c  a  b  a    M1

x1 x2 x3 x4  class
c  c  a  c  c    M2
c  a  c  a  a    M2
c  a  c  a  b    M2

x1 x2 x3 x4  class
c  a  b  c  a    M3
c  c  a  b  c    M3

x1 x2 x3 x4  class
c  b  b  c  c    M4

Thanks for your help.
Jan Sabee