ladette, i think it's as simple as four-month-olds just don't tend to have the co-ordination to self-feed so you kinda have to spoon purees (probably at 6 mos your lot were on fingers of toast etc). since the recommendation now is 6 months it allows us just to skip the puree, that's all.
sycamore, honestly, imo and ime they are little freaks. sometimes dd would gag something back into her mouth, pull it out with her hand, cackle maniacally, turn it round and put it straight back in again. weirdos. tbh they know what they're doing, they know what they're capable of.
i think that it's nevertheless unfair to put 'pressure' on mums to do BLW as they also know what they're capable of... for some people the mess is a no-no, they can't get over their (entirely reasonable) fear of choking or even gagging etc.
now it's all very well those of us who've done it saying 'don't worry' but some people do worry, that's just their personality type, so they're always going to be better off taking it slower with purees first. because it should be a pleasure, really, that's the main focus for me.