I too am having this problem. I was previously using an old version of FS Secure Contact without any problems at all, however when I upgraded to version 4 – I am limited by the number of fields. I have checked the PHP and can confirm that max_input_vars = 1000 memory limit is 90M.
My form currently has 60 fields, and it will not allow me to add any more. I need another 15 or 20 fields.
I have tried deactivating all plugins without success. Would appreciate any assistance you can give