Vlan & DHCP

So I have an issue I can' seem to get resolved.
This is the situation:

Firewal with DHCP range 68.x
On that same firewall VLAN50 that hands out range 88.x

We have a HP switch. The switch port 2 is tagged VLAN50

Now we have an ip phone (snom 720) with two ports. I will call it the Accessport and the PC port.
We want the accessport to get VLAN50 (range 88.x) en the pc port the normal Dhcp range 66.x
So we untagged port 4 and connected the ip phone to this. The ip phone doesnt have any settings for the moment.
It receives vlan50 and gets an 88.x address but unfortunatly the PC port gets same range.
In the Snom 720 (ip phone) we can enter a VLAN ID. Also for the PC port. If we enter a Vlan ID (50) in that field and reboot the phone he doesn't get any ip adress any more. I still struggling with the whole tag/untag.

My idea was to also put port 4 as tagged in VLAN50 and enter the vlan id 50 in the snom 720 vlan ID field.
Then leave the vlan ID PC port field empty.
But this also doesn't work. What am I missing here? How can this be accomplished?
Many thnx for anyone who can help me on the way...