Learn Portuguese (Portugal): Há quatro livros e cinco cadernos na mesa. - There are four books and five notebooks on the table.

No, the numbers from 3 to 10 are completely fixed. While the numbers 1 and 2 change depending on whether a word is masculine or feminine (like um vs. uma), quatro and cinco will always look exactly like this, whether you are counting masculine livros or feminine words like mesas.

Since there are multiple books and notebooks, why don't we use a plural form of há?

In English, we say "there is" for one thing and "there are" for multiple things. In Portuguese, the word há does both jobs! It never changes to a plural form when you are just stating that things exist in a location.

Why does the sentence use na to mean "on the"?

The word na is a contraction (a squished-together word) of em (which can mean "in", "on", or "at") and a (the feminine word for "the"). Because mesa (table) is a feminine word, Portuguese combines em + a into na.

How should I pronounce the word e (and) in the middle of this sentence?

When e means "and", it is pronounced similar to the "ee" in the English word "sheep", but slightly shorter. It does not sound like the English letter "A". In this sentence, it blends smoothly to link the two groups of items together.