The expression given is $(a + b)^3$, and we need to find the 4th term in its expansion.

The binomial expansion of $(a + b)^n$ is given by:

$(a + b)^n = \sum_{k=0}^{n} \binom{n}{k} a^{n-k} b^k$

For $(a + b)^3$, the expansion will be:

$(a + b)^3 = \binom{3}{0}a^3b^0 + \binom{3}{1}a^2b^1 + \binom{3}{2}a^1b^2 + \binom{3}{3}a^0b^3$

Simplifying, we get:

$(a + b)^3 = 1 \cdot a^3 + 3 \cdot a^2b + 3 \cdot ab^2 + 1 \cdot b^3$

So, the terms of the expansion are:

1. $a^3$
2. $3a^2b$
3. $3ab^2$
4. $b^3$

Therefore, the 4th term in the expansion is $\mathbf{b^3}$.

Would you like details on binomial coefficients, or have any questions?

Here are 5 related questions:

1. What is the 3rd term in the expansion of $(a - b)^4$?
2. How do you calculate the binomial coefficient $\binom{n}{k}$?
3. What is the general term formula in the binomial expansion of $(a + b)^n$?
4. How can the binomial theorem be applied to solve problems in probability?
5. How would the expansion differ if the expression were $(a + b)^5$?

Tip: In binomial expansions, each term is structured as a product of a binomial coefficient, one variable raised to a power, and the other variable raised to the complementary power.