Step 1: Total outcomes.
A die is thrown twice, so total outcomes are:
\[
6 \times 6 = 36
\]
Step 2: Favorable outcomes for first toss.
First toss should be 4, 5 or 6.
Number of favorable outcomes:
\[
3
\]
Step 3: Favorable outcomes for second toss.
Second toss should be 1, 2, 3 or 4.
Number of favorable outcomes:
\[
4
\]
Step 4: Total favorable outcomes.
\[
3 \times 4 = 12
\]
Step 5: Compute probability.
\[
P = \frac{12}{36}
\]
Step 6: Simplify.
\[
P = \frac{1}{3}
\]
Step 7: Final conclusion.
\[
\boxed{\frac{1}{3}}
\]