Ensure that the following is alone on a line: starting from the beginning of a line, enclose your subheading's text in 2 - 6 underscores. The number of underscores represents the level. (The level 1 heading is reserved for the page's title.)
____example Level 4 Heading____ =
If you fail to balance the number of underscores either side of your text, the lower number will be used, with the extra ones the other side being displayed as underscores.
No other wiki mark-up will be honoured within a heading.