Create a random subsample by ID and with a certain factor distribution in R

I am working with R and have the following dataset which consists of sentences taken out of books and contains data about the book id, their cover colour (colour), and a sentence ID which is matched with the corresponding book.

My dataset
    Book ID| sentence ID| Colour      | Sentences
    1      | 1          | Blue        | Text goes here
    1      | 2          | Blue        | Text goes here
    1      | 3          | Blue        | Text goes here
    2      | 4          | Red         | Text goes here
    2      | 5          | Red         | Text goes here
    3      | 6          | Green       | Text goes here
    4      | 7          | Orange      | Text goes here
    4      | 8          | Orange      | Text goes here
    4      | 9          | Orange      | Text goes here
    4      | 10         | Orange      | Text goes here
    4      | 11         | Orange      | Text goes here
    5      | 12         | Blue        | Text goes here
    5      | 13         | Blue        | Text goes here
    6      | 14         | Red         | Text goes here
    6      | 15         | Red         | Text goes here
    .

I would like to take four randomized subsamples (each containing 25% of the original data) with following conditions:

1) the distribution of book-colours should remain the same as in the original dataset. If there were 10% blue books, this should also be reflected in the subsamples

2) the subsample should not be taken/split by number of rows (which is the sentence ID) but by "Book ID". This means if Book ID 4 is sampled, then all sentences 7,8,9,10,11 should be in the sample dataset.

3) Also, each Book ID should only be in one of the 4 sub samples - this means if I decided to merge all 4 subsamples, I want to end up with the original dataset again.

What would be the best solution to split my dataset in the way described above?