Question: Say I have a function f(n) = 4n + log n, 
I know this would be in Θ (n), because with a large input, n or (4n) is much more significant than log n. It grows much slower. My question is that if I am not sure which side of the polynomial is more significant ( 4n or log n), how would I approach such a problem. You may use the same example or any other example to show me how.

Note: You may assume f(x) can be expressed in term of big Θ. 
Thanks a lot for the your effort, guys! 
I really appreciate your help.

Thanks again.

Answer: 
Among $4n$ and $\log(n)$, the *most* significant term is clearly $4n$, and that's because :

$$\lim\_{n\to\infty}\frac{\log(n)}{n}=0$$

Indeed, if you factor the whole expression by $4n$, you get :

$$4n+\log(n)=4n\left(1+\frac{\log(n)}{4n}\right)$$

and the parenthesis has limit $1$ as $n\to\infty$.
It is useful to memorize some common terms. Specifically, we have
$$
\Theta(1) < \Theta(\log \log n) < \Theta(\log n) < \Theta((\log n)^{c\_1}) < \Theta(n^{c\_2}) < \Theta(n) < \Theta(n\log n) < \Theta(n^2) < \Theta((c\_3)^n) < \Theta(n!)
$$
where $c\_1 > 1, 0 < c\_2 < 1$ and $c\_3 > 1$ are constants.