I want to browse a 4D matrix and I got this error message:
Undefined function '_colonobj' for input arguments of type 'double' and attributes 'full 4d real'.
The error message means, that in your code
the variable M is a 'full 4d real' (although I do not know, what this means). Check how M is defined and adjust it to be a double. Maybe this is working already:
[EDITED] Walter has explained the error message: M is a 4D array, but the colon operator expects a scalar. Then I guess you want:
for i = 1:size(radon_cartesien, 3) for j = 1:size(radon_cartesien, 4)