To find out how many books Micaela and her three friends collected altogether, we can calculate the total number of books given to the shelters.
There are 4 homeless shelters and each shelter received 18 books.
The total number of books is calculated as follows:
\[ \text{Total books} = \text{Number of shelters} \times \text{Books per shelter} = 4 \times 18 = 72 \]
Thus, Micaela and her friends collected a total of 72 books.