As I was saying, you have a server restrictions, in your case it's suhosin extension.
For starters play with the following settings in php.ini (or whatever setting file for that extension is)
suhosin.*.max_vars
and:
suhosin.post.max_array_depth
suhosin.post.max_array_index_length
suhosin.post.max_name_length
suhosin.post.max_totalname_length
suhosin.post.max_value_length
suhosin.post.max_vars
I've seen such restrictions before, when people were not able add more then NN number of images at once, your case looks the same.
Just to summarize, 4images sends out 21 POST vars, plus additional 11 vars for EACH image, so if you add one image, it uses 32 POST vars, 2 images = 43 vars
It also could be some other settings, but I'm quiet sure it's the suhosin extension that giving you the problem.