4. Petar said to his older brother Mark: "This year I will turn as many years old as the sum of the digits in the year of my birth." Mark replied: "Interesting, but the same is true for me." How old will the younger, and how old will the older brother be in this, the year 2019?
4. Let the birth year be $\overline{a b c d}$.

The person is $a+b+c+d$ years old.

It is given that $\overline{a b c \bar{d}}+a+b+c+d=2019$

$1000 a+100 b+10 c+d+a+b+c+d=2019$

$1001 a+101 b+11 c+2 d=2019$

The number $a$ can only be 1 or 2.

Let $a=1$.

Then we have:

$1001+101 b+11 c+2 d=2019$

$101 b+11 c+2 d=1018$

The number $b$ can only be 9.

From $b=9$ we get:

$909+11 c+2 d=1018$

$11 c+2 d=109$

The number $c$ can only be 9.

Then we have:

$99+2 d=109$

$2 d=10$

$d=5$.

The person was born in 1995.

The second case is obtained for $a=2$.

Then we have:

$2002+101 b+11 c+2 d=2019$

$101 b+11 c+2 d=17$

The number $b$ must be 0.

From $b=0$ we get:

$11 c+2 d=17$

The number $c$ can be 0 or 1.

For $c=0$, we have $2 d=17$, which does not yield a natural number as a solution.

For $c=1$ we get:

$11+2 d=17$

$2 d=6$

$d=3$.

The person was born in 2013.

This year, 2019, the older brother is 24 years old, and the younger one is 6 years old.