The N4 uses HSPA technology, what that means in terms of speed depends on which carrier you're using. You don't need to do anything you except 'maybe' update the APN settings on certain carriers.
LTE is not supported officially, but the N4s chip does have some limited LTE capability. Only 1 specific frequency's is available, so you're limited to just T-Mobile in the US and a couple carriers in Canada. You can hack together a way to enable it, but it involves a lot of potential and unexpected tradeoffs.
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