6 months. The reason being (besides that's what the guidelines say and I think those are definitely important to take into consideration in deciding what you personally want to do) is because we wanted to do baby-led weaning. BLW is awesome and it was such a pleasure to let her explore food on her own terms. We now have a very adventurous 2 year old who eats all sorts of things that kids her age don't normally like (except for bland food and anything breaded and fried, like a fish finger, she won't touch those and I have no complaints about that!). I think BLW is so fantastic and you really can't do that at 4 months, so I'd never sacrifice that for the choice to wean early.
Also, it wasn't a deciding factor at the time, but looking back I'd say one of the main reasons I'm so glad we didn't wean early was simply because it gave us more time for just doing milk feeds. Weaning, if you're going to offer healthy, home-cooked meals, takes time and money and effort. Why make more work for yourself when you're finally hitting the time when you might be getting a bit more sleep, your baby is more interactive, and you could be focusing on having fun with them? You'll be slaving away over healthy family meals for all of you for the next 18 years. There's no reason to rush it. I'm glad we had just a few more months when life was a little less complicated. If it was a rough day and I was exhausted, we could just get a frozen pizza. I didn't have to worry about what I'd feed her. And we got away with spending a little bit less on fresh food. I didn't have to stress about bringing food with me when we visited friends or family (who never seem to want to eat a meal at the same time my daughter is hungry, so I'm always preparing her an early lunch in other people's kitchens when we visit) and I didn't have to pack up healthy snacks for a day out. We could just bring a bottle and it was easy. Weaning just adds an extra layer of complexity to doing anything. I get that people get really excited about it and I think that's why so many people still start early. But I think sit back, relax, enjoy your baby just being a baby for a couple more months. Besides just being what's recommended anyway for a host of medical reasons, it's just nicer and more enjoyable too, especially if you eventually go down the BLW route when they're ready.