Due to the size of the house and the heating and hot water demand, it would be impractical to fit a combination boiler, I will assume that a 4 bedroom house has 5 people living in it, 2 adults, 3 children.
On average a house will use approximately 135 litres of hot water a day, plus 40 for every occupant.
That's a lot of hot water demand on its own.
Then there's the fact that you have to take into account the construction of the building, do you have double glazing, is everything insulated as it should be etc etc and the frequency of use, if the hot water is used often, for example it's a house full of girls or women and the bath or shower is run constantly your boiler is going to be on hot water most of the time, and the house will be colder as a combination boiler will only have one function running at any one time. This means that when the hot water isn't on demand, the boiler will still be working hard to hear the home. Harder than it should normally have to.
Granted there's no recovery time for the hot water via a combination boiler and this might suit you, but if the boiler is undersized, you can expect a dramatically reduced life from it. Every house is different with different needs, without seeing building plans or the state of occupancy nobody on here would be able to give you a comprehensive answer.
It is not impossible to fit a combination boiler, but the system that was in the house before was designed for your property, what is the reason for change? are you struggling to meet the hot water demand, is the boiler broken or beyond a state of economical repair? A combi boiler is extremely effiencient if it is installed in a situation that allows it to be.
A 1.0 corsa is efficient until you take it on the motorway and make it do 70mph.
If you would like my telephone number to ring me, private message me and I will give you it, failing that reply to this and I will try and give you the best answer I can.
Kind Regards,
Jordan Murray - Gas Engineer