[css-box-4] Correct version/level metadata (#4882)

* [css-box-4] Correct version/level metadata

* box-4 has no previous version

Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>

Author: sideshowbarker

css-box-4/Overview.bs

@@ -1,13 +1,12 @@
 <pre class='metadata'>
-Title: CSS Box Model Module Level 3
+Title: CSS Box Model Module Level 4
 Shortname: css-box
-Level: 3
+Level: 4
 Status: ED
 Work Status: Exploring
 Group: csswg
-ED: https://drafts.csswg.org/css-box-3/
-TR: https://www.w3.org/TR/css-box-3/
-Previous Version: https://www.w3.org/TR/2018/WD-css-box-3-20180802/
+ED: https://drafts.csswg.org/css-box-4/
+TR: https://www.w3.org/TR/css-box-4/
 Abstract: This specification describes the margin and padding properties, which create spacing in and around a CSS box. It may later be extended to include borders (currently described in [[css-backgrounds-3]]).
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Ignored Terms: internal table elements, block layout