Actually in 5E a singe Wraith vs a group of 6 level 4 characters is considered an easy encounter. 2 Wraiths though ramps the difficulty up to Deadly.
You can use this tool here. It looks nice but I really can't attest to its accuracy since I didn't find it until tonight.
http://kobold.club/fight/#/encounter-builder
You can use this tool here. It looks nice but I really can't attest to its accuracy since I didn't find it until tonight.
http://kobold.club/fight/#/encounter-builder