2 Datacenter sharding advice wanted

We are currenty running out of one datacenter but will soon be moving
to another and our current datacenter will become the failover
location.

Currently running 2 shards made up of a replica set of 3 server each.

Just looking for an experienced opinion here to tell me if I am way
off in the bush or if my plans look sane.

I was thinking of adding a 4th member to the replica sets as well as
an arbiter to bring the total to 5 votes in each set.

Then I am going to move 2 of the set members and the arbiter instances
to the new primary datacenter.  Will also move 2 of the config servers
to the primary datacenter.

So in the end it will look something like this:
Datacenter A (primary):
Shard A - member 1
Shard A - member 2
Shard A - Arbiter
Shard B - member 1
Shard B - member 2
Shard B - Arbiter
Config 1
Config 2

Datacenter B (failover):
Shard A - member 3
Shard A - member 4
Shard B - member 3
Shard B - member 4
Config 3

On failover (which should never happen) the arbiter server would be
powered on in DataCenter B via VMWare SRM and all should be happy
other then only one config server being available so the shard config
would be frozen but that's fine since we'll be happy enough to even be
up.

As another note the Arbiter servers in Datacenter A will not be able
to talk directly to the set members in Datacenter B due to the
arbiters being in a special SRM network that exists in both
Datacenters.

Opinions welcome,