4 or 5 is OK if the children are keen and motivated, and you have a teacher who is enthusiastic about teaching that age.
DS2 (nearly 6) plays a bit, but his enthusiasm comes in fits and starts. (I play quite well, so am happy to teach him the basics myself.) But there's no way he currently has sufficient motivation/discipline to practice regularly that would make lessons a worthwhile investment IMO. I'm hoping he'll be ready soon, but suspect it will be at least another year before he is.
But there's lots you can do before lessons if they are not at that stage. Ds2 can read music and has learned all the note values, and loves writing songs!
I teach piano. You really want a child to be able to read as it gives them all teh necessary skills to be able to read music. It's all quite complex business really - following the music; knowing what it all means and interpreting it onto your hands... if you have to learn all the necessary skills to help with the reading from new, progress is going to be quite slow. You can certainly give practical experience and if they have any natural talent it could be beneficial, but for regular learning in teh conventional sense, being able to read (follow and decode a few sentences of text in a simple book) is the usual determining factor for learning to play. HTH
I started at five, i did ask for the lessons, i guess a child of that age has to "want" them.
How about doing a trial run and see what dd thinks? I would suggest booking 3, because i can remember my first one being terrifying (going into a strangers house etc) so 3 would give her the opportunity to relax in the environment and decide if she liked it. I carried on until i was 15 and now wish wish wish our house was big enough to fit a piano for me and my dd
I wouldn't rush it tbh. I don't know how old your dd is, but before 6 it really is difficult (I have taught one or two 5 year olds, and they have found it very hard going). Hand size can be a limiting factor - I have had one boy with minute fingers who found it very hard to even do 5 finger tunes. If she can count to 5, then you can do some simple things at home with her - put both thumbs on middle C, then you have 1-5 in both hands (with thumb being 1, little finger 5) and lots of tunes can be played using numbers. At the beginning, the most important thing is that they don't get disillusioned, because they will almost certainly get on faster at age 7 or 8. But if she's showing an interest, might be a good time to learn the basics.
That's a bit rambling - I guess what I'm saying is that before about age 6 it might be best to do some at home with her, rather than the pressure of a weekly lesson.
If she knows her letters A-G (and the order) then you could also do a bit of music reading - I learnt to read music when I was 3, at the same time as learning to read.