It's your legal right to defer school entry until compulsory school age - which is five years old. I did this for my son, and he'll start reception at the beginning of the (final) Summer term in April.
It wasn't a simple process - but it should be. Basically, the School Admissions Code 2.16 (b) clearly states that it is for the parent to decide if they want to defer until compulsory school age.
Compulsory school age is set out in section 8 of the Education Act 1996 and the Education (Start of Compulsory School Age) Order 1998. A child reaches compulsory school age on the prescribed day following his or her fifth birthday (or on his or her fifth birthday if it falls on a prescribed day). The prescribed days are 31 December, 31 March and 31 August.
My son's 5th birthday is 18 February and so his 'prescribed day' is 31 March. Hence, he starts in the final term.
I applied for primary school in the usual way, and accepted a place at our chosen state school; your allocated school has to hold that place until compulsory school age. My headmaster had never dealt with this situation before. My LEA originally said that my son could only defer until the January term - until I provided them with above info and written back up from the Department for Education.
I feel four is too young and as a family we'll have spent a wonderful extra six months together. Precious time. Deferring is not for everyone (or every child), but I do wish your rights on this issue were made clearer. The main reaction I get is: 'Oh, I didn't know you could do that'!