If you only put in your boiler four years ago I would be surprised if it was cost-economic to already replace it, it must be quite a new one?!. We put ours on 5 years ago (new built house) and it is 95% efficient, I think you cannot get better than that. Yes our oil costs have gone up too, but spending large amounts of money again so soon would not be justifyable (for us). But we also put in a multifuel burner in our large living/dining/kitchen area (I think it's 8kW), and that does us for heating most of the year. We also use coal on it when it is about -20 outside. (Central only goes on for a couple of hours first thing in the morning, again for an hour at noon and maybe for an hour pm, but I usually switch that off as the fire will be on.)
We are thinking of putting some solar water heaters onto our south facing roof, so we can stop using oil from March to September. (and not have cold showers...)
Maybe you want to look into ways of reducing your oil use rather than completely change the system?