As a Reception teacher, I would be horrified if my school did this. It's not acceptable. Ebooks can
provide additional reading for those who can access them and want to access them, but they absolutely should be the only reading sent home.
I understand the people who are making excuses for the school by commenting on school budgets are coming from a good, understanding place. You are indeed right that school budgets are shockingly low these days and many schools are struggling to provide the basics. However, physical scheme books are a must and that is a hill I will die on. This is not ok.
It sounds like this school are taking 'fidelity to the scheme' literally. I'm guessing they've started a new scheme, and have decided they can't afford to buy all the books. They've wrongly interpreted Ofsted's 'fidelity to the scheme' guidance as they are unable to send home any book which isn't Collins Big Cat phonics. This isn't correct. Children should (according to Ofsted - not my personal opinion) only receive reading books at their current phonics level, containing only taught sounds. There is nothing to say home reading books can't be supplimented with other phonics books the school own, as long as they only contain sounds the child has been taught. The companies themselves will tell schools otherwise. Of course they will, they want to make money out of them. Ofsted see nothing wrong with mixing schemes for home reading, as long as the child has been taught all the sounds covered in the book.
We send home a variety of scheme books. My school doesn't teach Little Wandle, but we send home several of the Big Cat books they use early on because we have them and it would be wasteful not to use them any more. They contain exactly the same sounds we're teaching at this very early stage, so it's fine.
There are ways and means and this school are failing their Reception children if they're not providing any physical reading books.
OP, explain to the teacher that you won't be able to read the EBooks as you don't have a tablet and have no intention of buying one for your 4 year old. Take it to the Headteacher if you need to. The school need to hear from as many parents as possible who are not ok with this in order to change their mind. You'll likely find that the teacher is in complete agreement with you.
I don't agree that parents should have to source their own scheme books for their children, but I do agree with people saying check the library, charity shops or Vinted if you wish to do it as cheaply as possible (you could always sell them on again in a few months to fund the next level, and so on). Please take it from an experienced Reception teacher, it really doesn't matter what scheme you read (no matter what the school and Ofsted say). They are all much of a muchness.