I think that this is one of those cases where it depends on personal preference and learning style.
The reason I say this is because the ability to shift from one task to another (also known as 'set-shifting') is a type of cognitive flexibility that is linked to one's individual executive functioning ability. In turn, this is usually mandated by one's frontal inhibitory activity.
On the other hand, those who often fall prey to a phenomenon called task perseveration (also known as 'hyperfocus') may have a reduced or impaired ability to switch between tasks. This seems to occur at above-average rates in twice-exceptional children. [1] For those predisposed to task perseveration, switching between several topics a day may be marginally more difficult.
Another thing to consider would be cognitive load, which is the amount of mental effort being used in one's working memory. As cognitive load increases, it becomes difficult to manage and learn new material. It is known that cognitive load increases in the face of difficult material (with 'difficulty' as perceived by the learner). For this reason, one common method of reducing cognitive load in teaching and learning is to divide subjects up into subtopics and teach these subtopics in isolation, and then later integrate them into the subject as a whole. Furthermore, it is known that there are individual differences in cognitive load capacity, particularly among experts and novices. [2] Experts have more knowledge associated with a topic or skill, which reduces the cognitive load necessary to learn more material within that topic. This lends itself to the idea that the more familiar you are with the material, the easier it becomes to learn more of said material. [3]
In short, there is no clear-cut answer; there are simply factors that may go into a contextualized answer. In the case of studying four unrelated subjects, it might be worth asking the following: How well do you know the subjects? Are you studying all four from the ground up? If so, it might be more effective to study one at a time in order to allow the fundamentals to sink in, thus decreasing cognitive load. The other factor to consider would be your overall discipline level. Do you fall prey to task perseveration, or is your ability to shift from one task to another generally uninhibited? These are all factors to consider in determining your own study plan.
Sources used:
[1] Misdiagnosis And Dual Diagnoses Of Gifted Children And Adults p.50-51
[2] Murphy, Gregory L.; Wright, Jack C. (1984). "Changes in conceptual structure with expertise: Differences between real-world experts and novices.". Journal of Experimental Psychology: Learning, Memory, and Cognition 10 (1): 144–155.
[3] Voorhies, D. & Scandura, J.M. (1977). "7". Determination of memory load in information processing. Problem Solving, NY: Academic Press. pp. 299–316.
