if your 10.4.x.x is connected to a port on 6509 and your 10.46.x.x is connected to another port, yes you can create a NAT on 6509. But this NAT should cover both networks.
what you also can do is, on the router module of 6509 and 5500, configure an EIGRP (or RIP2) and subnet 10.46.x.x with VLSM (in other words extend the current subnetmask, 255.255.0.0, a few bits) so you can have 2 subnets within the subnet 10.46.x.x remember only your 6509 and 5500 are supposed to know this new subnet mask.
If I were in the situation, I would use VLSM because NAT costs a lot of CPU resourse.
I have a discription of subneting subnets in Heather's thread "vslm" in this forum.