4. The sum of a three-digit number and a two-digit number containing 3 tens can only result in a four-digit number if the hundreds place of the three-digit number is 9. In this case, the digit in the thousands place of the four-digit number will be 1.

$$
\begin{array}{r}
9 X 4 \\
+\quad 3 X \\
\hline 1 X X X
\end{array}
$$

The digit in the tens place of the three-digit number must be such that when added to 3 or 4 (due to the sum of the units being greater than ten), the result is a number greater than or equal to 10. These numbers are $6,7,8,9$.

Answer: $964,974,984,994$.
4. What number can the first addend be? Justify your answer.

$$
\begin{array}{r}
\mathrm{XX} 4 \\
+\quad 3 \mathrm{X} \\
\hline \mathrm{XXXX}
\end{array}
$$