The following code seems to work fine both in IE and firefox for me, seems if percentages are used it doesn't work correctly.
<div style='width:400px; display:block; text-align:center; '>
<span style='font-weight:bold; width:80px;float:left;'>Col 1</span>
<span style='font-weight:bold; width:80px;float:left;'>Col 2</span>
<span style='font-weight:bold; width:80px;float:left;'>Col 3</span>
<span style='font-weight:bold; width:80px;float:left;'>Col 4</span>
</div>