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Is it better to have 4 troops and 5 heavies, or 5 troops and 4 heavies for an Eldar army

Depend on the units.

But I'd say 4 troops is the minimum. I'd actually prefer 5 or even 6.

Well in a double force Org isn't 4 troops the min to unlock double FOC?

It also really comes down to what you're bringing like what jy2 said.
4 squads of 10 guardians != 4 squads of 10 wraithguards