SAS Proc Plan Random Block Size for Randomization List - Or how to do in R?

Question

I am trying to create a randomziation list with PROC PLAN and I am trying to figure out how to do a random block design (that is, the block size changes randomly). I know how to set a block size, but I would like the block size to change so it is unknown what group the last participant will be placed in (for example, if I know the block size is 4 and my first three participants are places in ABA, I know the 4th will be placed in B. Having random blocks will eliminate this).

Here is example of the code I am trying to manipulate:

proc plan seed=12345;

factors blocknum = 10 random treat = 4 random;

output out = list treat nvals=(1 1 0 0) random;

run;

What I would like to do is so that 4 changes (aka to 8, 12, etc.) each time where particiapnts will still be allocated to either group at a 1:1 ratio. Thank you for any help I may recieve!

ALSO If this could be done in R, that would be great too!