4a is the same as adding "a" to itself 4 times.
So, 4a = a + a + a + a
Since 4a and a both have the term "a" in them,
4a + a = a + a + a + a + a
Since there are 5 a's on the right side, this can be combined to simplify,
4a + a = a + a + a + a + a
= 5a
