please try steps bellow to rollback 3.8.4 firmware, which had standard version surveillance station, that you can use all 4 cameras again.

notice: your cameras must be in support list Surveillance Station (NOT pro)
http://www.qnap.com/en/index.php?lang=en&sn=4056

1. REMOVE all qpkgs in app center.

2. please reboot your nas and follow steps bellow to rollback your firmware.
download firmware file here and unzip it
http://www.qnap.com/v3/en/product_x_down/
download Qfinder and install your pc
http://download.qnap.com/Storage/Utility/QNAPQfinderWindows-4.0.1.0530.exe
when you running Qfinder you suppose to see nas in Qfinder
using Qfinder to update your nas firmware (tools->firmware update)


notice: reboot nas before you update firmware, that can help nas release enough memory