where should pin 6 of usb blaster ii cable to connect for FPGA max 10 and Cyclone IV?
Using JTAG mode, pin 6 proc_rst
Should it be some pin user defined for system reset?
Any dedicate pin of FPGA?
Yes, it is a pin used to trigger warm reset of the HPS block when prompted via the ARM DS-5 debugger in JTAG mode, It depends whether if you are using that or not. Else you may leave it unconnected.
As per the note in USB Blaster II user guide, you are recommended to connect this pin to a secondary device such as the MAX V CPLD, and use the device to manage the reset network for HPS.