Like a lot of symbols in Death in Venice, the sun motif recalls the philosophy and mythology of ancient Greece. Let's start by taking a look at the beginning of Chapter 4.
Day after day now the god with the flaming cheeks soared upward naked, driving his team of four fire-breathing horses through heaven's acres, his yellow ringlets fluttering wild in the gale of the east wind. (4.1)
Okay, it's pretty clear that the narrator is talking about the sun here, which has finally appeared in Venice after days of overcast weather. But there's more going on here. By calling the sun a "god" with "fire-breathing horses," the narrator is referring to Helios, the Greek god of the sun, driving his chariot across the sky. This is the start of all the associations that will link Venice to the classical world.
Consider this, though: It's emphasized that the "god" is "naked," and that his "yellow ringlets" are "fluttering wild." Does that remind us of anyone? Well, Tadzio is also described as having a face "ringed by honey-colored hair." (3.40). And while Tadzio might not appear naked in the novella, certainly this erotic image is in line with Aschenbach's, er, motives.
And there's more still: In Plato's Republic,one of the most famous works of ancient Greek philosophy, the sun appears as a symbol for Truth and the Good. The metaphorical "light" of Truth gives form and meaning to all things, just as the sun reveals the shapes and forms of earthly things. But there's a catch: Like the sun, looking right at the Truth can blind you, so sometimes the best way to access Truth is by considering its many reflections in beautiful forms.
Tadzio, in addition to appearing like the sun god Helios, is described as Aschenbach's "effigy and mirror" (4.8), perhaps suggesting that his physical beauty, in the Platonic sense, reflects and embodies some philosophical Truth or Higher Good. The question then is whether Tadzio reflects too much of the Truth, becoming, in fact, a mirror in which Aschenbach confronts his own dark, lustful, and self-destructive impulses.