HTML 4 was introduced in 1997 (yeah, a long time ago). The biggest gain in HTML 4 was the separation of formatting into CSS. Formatting tags like <font>, <s> and <u> and attributes like bgcolor, height and width were deprecated in favor of corresponding CSS properties. This allowed for big gains in accessibility and much easier, more semantic mark up (e.g., tableless layout).
There were also additional accessibility improvements, including requiring the alt attribute on <img> tags and allowing for the title attribute on almost all tags.
It took browsers awhile to conform, but eventually they managed and now web designers can safely take advantage of most HTML 4 / CSS features.