Radon Transform reducing the number of parallel beams (MATLAB)
Unfortunately you can't do this with the radon function as it comes.  However, the function is written in MATLAB form (as opposed to being a compiled function), so you could extend the function yourself.  This would also be a good way to learn more about the process.

To reconstruct properly, you would also need to perform edits on the iradon function.

An alternative way to get what you're looking for would be to get projections as they come from radon, then reduce the array size by averaging the projection rays in groups of 4.  You would probably want the reconstruction to be the same size, so you'd need to repeat the averaged values 4 times in the argument to iradon.  So maybe something like this:

p = phantom ();
proj = radon (p, 0:179);
proj_avg = proj (1:4:end,:) + proj (2:4:end,:) ...
         + proj (3:4:end,:) + proj (4:4:end,:);
proj_rep = reshape (repmat (proj_avg',1,4)', [], length (proj(:,1)))';
recon_few_beams = iradon (proj_rep, 0:179);