I had the same dilemma (birthday in two days!), but I am so, so glad I sent her into Reception at just-turned-four and didn't defer it. Yes, she's still that little bit behind, yes, it's been pretty hard going at times, and yes, she's absolutely exhausted now, BUT the difference between her and the children from nursery who are just a few weeks younger is actually quite big. Even if they don't leave reception with all the academic stuff under their belt, they leave with a lot more maturity (well, as much as an almost 5yo can have!), independence and discipline than either keeping them at home or in nursery for the extra year. And, most importantly, at least for the State schools, you can't just keep them back for the year and send them into Reception the next year, effectively making them the oldest in the year - they have to go into their rightful year, which means going straight into Year 1. That was what clinched it for me. I didn't want her to miss out on the important ground work which is laid in the reception year. The academics, for me, haven't been as important as the preparation for the "real" curriculum starting in September - she'll read, write, etc when she's ready for it to "click", and once that click happens, she'll catch up quickly. They catch up, but it just takes a little time for them to reach that point of maturity. The almost year difference between eldest and youngest is massive when you have children who start in September and turn 5 straight away, but others who have only just had their fourth birthday - they're still babies! I'm sure that gap will gradually close, though. And, as my daughter's teacher told us, she's a June baby herself and she's done very well, thank you very much!
Your summer baby will be fine, but just don't be disappointed if they need a bit extra time to master some of the skills the older ones (or even the ones the same age!) get quite quickly. Nurture other talents in the meantime and everything will be fine