It would make light travel a whopping one block further in the three directions you add the lava to.
Given the lava physics in Minecraft, light intensity and radius are the same. A candle gives 14 luminance and 14 radius. Lava and sunlight have 15 luminance instead.
Here's how light would spread if lava had 5 luminance:
Here's the same with a 2×2 source:
...but that's a waste. Here's what the same amount of resources (4 lava sources, 12 glass columns) can get you:
12321212321 + 1 lava source
123G321 (This is just an
123GLG321 example, you can
123G321 probably do
12321 better than this.)
Also be aware that light also spreads out vertically: a light source one square above ground level is what will give maximum luminance at ground level. Light any higher or any lower will give you less luminance, as light has to travel through more air to reach the ground.
Thus, the height of the pillar doesn't matter if all you care about is mobs not spawning.
Remember each level of the 16 levels of luminance (0-15) appears to 20% darker than the other.