three events in the same letter? i'd probably have considered three separate letters to make it somewhat of a three act thing. because it's 'only' 10k words, i think you've got some leeway there to play around with POV's, whereas a novel might make it annoyingly 'cute' (at least to me). i agree, 1st person from a 4 yr. old's perspective... ah... well... i'd have to wonder how much depth you could extract from that. true, the opinions stated by a four year old are brutally honest, but they're still the opinions of a four year old nonetheless. for my money, i want the awareness and cognitive ability of an adult rather than the perhaps misconceived perceptions of a toddler. besides, the misconceived perceptions of an adult lends far more subtext to an adult character than a child's, i think.
flashbacks in this case might work well in 1st person. it would help personalize that line of the narrative. still, i'd consider three letters or breaking the letter up into three sections and structuring the POV's around that for consistency. i disagree, though, with it being 'jarring' keeping the flashbacks in 3rd person. i did prit near a book centred around diary excerpts from 1692 and the narrative of that time was in third person, no problems (book lost in computer related disasterosa). i think the point is not making the story read as if it were an experiment or writing excercise, which mixed POV's read like to me. (before anyone declares, 'well, so-and-so-famous/gifted author did it,' that argument doesn't fly for me. unless someone's talents are of that calibre, it reads like shyte more often than not. besides, picking out four or five examples out of a million doesn't exactly prove much other than it can possibly be done, has been done... and yet how often is it successfully done in the 21st century? mixing POV's is just an immediate turn-off for me, as it probably is for the majority of actual readers (i.e. non-writers not trying to prove a lame point by name-dropping books they may have never even read, which just would prove my point further).)
but, if there's an actual reason to change into 1st person, well, i guess it could work out. but, if there's nothing really to be gained, one might surmize early on that the shifts are trying to hide a weak story. you're right, too, in that someone might get cornfuddled between 1st person flashbacks and the letter. you could off-set the letter with margins, but that's kind of a pain in the arse. on the flip-side, who's to say a reader wouldn't cornfuddle the flashbacks in third person with the current POV.
as an aside, i could see a letter being read aloud to a small group of people and there happens a round-robin (is that the term i want?) of first person flashbacks. anyway, as it sounds, i'd probably stick with basic flashbacks and not worry about getting fancy. i would, however, most likely parse out the letter in sections as opposed to one big gulp. that gives it a framework to drape the rest of the story on, eh?
remember the movie, 'green fried tomatoes'? didn't that have something rather like what you're wanting to do? i admit, i'm not really sure what you're proposing, but i think that's the template i'd be aiming for. the problem is that usually when you've got a first person flashback, that person is in the present, so, if mum's not around to do the flashbacking, how can you legitimately do that? i might do the whole thing in first person and let the flashbacks be third person instead of the other way around.
hope that's some food for thought, sorry for the rant, heh heh.