It's unlikely a 15 year old boiler would be that inefficient.
I think the regs making condensing boilers mandatory were around 2005. This means it is unlikely your boiler is non condensing - but I guess it is possible.
You'd have to provide the boiler model number/identification for it to be confirmed.
My guess is that a boiler of that age might be 20% less efficient than a new boiler, so not a huge amount.
To me it's far more likely that a) you are using a huge amount of gas b) that you have a billing mix up or c) you have a units mix up.
In order to be able to figure out you need to do some detective work, and if you want people on here to help you need to post a lot more detail. To start :
i) How much of your 4000 kWh per month is electric and how much gas ?
ii) How many hours do you have the heating on per day ?
iii) What does the meter display in units (ft3x100 or m3) and what does your bill say for the units used ?
iv) What is the change in value of the actual meter over a well defined period ? For example read your meter in the morning, and then the next day at the same time to get your usage in units and we can help calculate the actual kWh.
People on here can help you get to the bottom of it, provided you post the information they request.