How can I decide on taking four classes for first year in graduate school? I just finished undergraduate degree from a mediocre liberal arts college. I am attending a graduate school ranked about 30. I feel that I am lacking a little in my basic knowledge. Therefore, I was thinking about taking three to four classes in basic subjects (two semesters each of algebra, real analysis, complex analysis and algebraic topology) during my first year. 

Coming from a liberal arts college, I have never taken four classes in a single semester and my plan is to do exactly that for two consecutive semesters. 

If it helps, those four classes would be 16 credits in a semester. Algebra and analysis are 5.0 credits each. Complex analysis and algebraic topology are 3.0 credits each. 

Also I am on a fellowship for first year of my graduate school, which means I do not have to teach and can dedicate my time for studies. 

How can I approach this decision? What aspect may I not have taken into account?
In the U.S., to have a fellowship as opposed to teaching assistantship really does free up considerable time each week, so four classes rather than the norm of three is completely plausible, if that's the way you decide to go.

I would want to strongly disagree with some of the claims, in comments above, that you'll never use most of the Prelim-level material "in your research". This is not naturally true, but can certainly be made true by being sufficiently ignorant, and/or sufficiently misconstruing the material in the Prelims and corresponding courses. (Also, one may encounter caricatures of Prelim exams.)

I would also strongly disagree with the claim that a (typical) undergrad degree in math (in the U.S.) provides sufficient breadth. This is even more crazily false than the supposed irrelevance of Prelim exam material. But, as with most things, clever people can contrive arguments for or against almost anything.

The genuine question is whether you should do four "core" courses, rather than take "electives", and/or self-study or group-study. First, this obviously depends on how effective/disciplined you are at self-study. If "very", then this is probably optimal to some degree, because it can be more flexibly scheduled, if nothing else. If you do better in a group setting, or with an instructor organizing things and making a schedule, then coursework might be better. 

But there does remain the question of whether or not to try to do all the basic things (by which I don't mean standard U.S. undergrad math) prior to everything else. I generally don't think this is a good idea. Instead of all four basic grad courses, maybe do three and an "elective" in the/a direction of your curiosity. Maybe with less homework, and less "adversarial" feeling.

Over the years, I have seen many people "get tired" from (arguably over-assigned) homework, to the point that they have forgotten why they came to grad school, have forgotten their affection for math, and have lost a sense of genuine curiosity, from being "over-driven".

For that matter, much of the "basic" material is deemed essential-for-everyone (not only for general cultural sense, but) because it comes up all the time. (Sure, there is tradition, too, whose inertia sometimes swamps sense.) If/when people (especially novices, e.g, grad students) tell you that you "don't need to know" various things, you really should not believe them too strongly (because they probably don't know what they're talking about, or else, if nothing else, they'd have subtler opinions about it), even if they are your friends and/or peers.

It is true that thinking in terms of "research", especially if this means something opposite to the artificially over-ordered world-view of textbooks and established curriculum (and grades, and schedules), is a very good thing. Genuine mathematics is not as highly ordered/structured as one might imagine from textbooks and standardized course concepts. Nor are the questions/issues at the end of any particular chapter, so that one knows the relevant theorems! It is also not "graded". So, yes, experiencing something opposite to classwork is very good... but this is absolutely not an either/or.

That is, whether you acquire basics by self-study or guided study or coursework, that is just one thread. There is at least one other, the one where you make the decisions, based on your curiosity, etc, and are not graded or judged. We can call this "research", but that word has much other baggage these days, which is not immediately relevant to the practice of mathematics.