4 bedroom houses typically appeal to large families or house sharing students and young professionals. Typically, the largest volumes of 4 bedroom houses can be found outside of major metropolitan areas where there is space for development and it would generally be unusual not to have a garden with a house of this size.
If you're thinking of buying a 4 bed pad, you should expect 1,500 square feet of floor space. Obviously you get less for your money in cities like London, Manchester and Birmingham, but even outside of London, be prepared to pay north of £400k for a property of this size.