So, this is a pretty complicated topic. Your reasoning is actually pretty good, but you are missing some key data. Comparing a phenol to a carboxylic acid is somewhat hard.
The first, simplest, and least satisfying answer is that the $\ce{NO2}$ is farther away from the acidic hydrogen in 4-nitrophenol than the oxygen is from the acidic hydrogen in acetic acid. Not good enough? Let's dig further.
We might decide to expand upon your argument that because 4-nitrophenol has one more oxygen in it than acetic acid, there is more potential for electron withdrawal, and hence 4-nitrophenol should be stronger. Solid reasoning, but consider our first, less satisfying answer. Both of these effects might be combined, and we could reason that the distance effect is stronger than the increase in electron withdrawing power. Decent reasoning, but still unsatisfying.
Enter the Hammett Equation:
$$\log \frac{K}{K_0} = \sigma \rho$$
Fully explaining this takes up one or two chapters of a graduate level physical chemistry book. Quick summary, $K_0$ refers to a reference reaction, which has hydrogen as its substituent. $\sigma$ is the electron withdrawing(positive) or electron donating (negative) value of a substituent replacing the hydrogen. $\rho$ refers to a parameter that is descriptive of the reaction itself. The parameters are empirically determined relative to the acidity of substituted benzoic acids. When benzene rings are not involved, sometimes an equivalent called the Taft parameter is used. Run 6 reactions (5 plus reference), draw a line, get your rho value for any reaction.
However, the reference reaction is the dissociation of benzoic acid. $\rho$ is different for the dissociation of acetic acid and for nitrophenol. So, even though nitro is a much stronger withdrawing group, its influence is mitigated by a reaction much less subject to such effects. The Wikipedia article is not terrible on this matter.