...my question is if the following setup is good, or if anyone have a better solution:
Synology NAS with 4x4 TB hds in raid 5
This sounds like a reasonably good setup. What is needed for editing is a fast data stream and a 4 drive RAID should give that to you. My preference today since I use a lot of multi-cam, would be a 6 drive RAID over Thunderbolt. Use at least 7200 rpm drives. Use the RAID strictly for serving media. Network speed is crucial; I prefer to work with either Thunderbolt or eSATA connected RAIDs
Three 4TB usb drives connected to NAS for backup
sounds good - just set the backup to work when you are not editing
One 1000 GB ssd used as project drive
The project file is good to have on your boot drive. 1000 GB is massive and would allow you lots of room. I work off a 250 GB ssd boot drive which I keep at no more than 50% full. The project file resides there.
One 1 TB usb drive as backup for project drive
Again, sensible and adequate
Can it be done cheaper in any way? And do i need an ssd for project drive? ...
It probably might be done cheaper depending on the size requirements of your media, but I do not see any obvious area to cut back other than the size of the SSD. I would set preferences to write any renders to the outboard RAID so that you are not clogging the SSD with rendered media