hyperlink text in word 2010 document to another header

The following is based on the assumption that you've already set up headings (e.g. Heading 1, Heading 2) that include numbering, so that the 4.2.5 in your example will get updated automatically, when you add or remove a section.

The feature you are looking for is called Cross-reference.

1. Position the cursor just after "as seen in section"
2. Select the References tab of the Ribbon

3. Click on Cross-reference in Captions

   This brings up the Cross-reference dialog:

   

4. Be sure to change the Reference Type to Heading

5. In the list, click on the heading you want to link to
6. Set your other options, as desired, e.g. whether you want to Insert a hyperlink and - in Insert reference to - what text you want to be displayed, e.g. just the Heading number (which it sounds like you want), or something more detailed, e.g. "Heading text"
7. (Obviously) Click Insert

Other useful info:

• If any of the linked text changes, Select All, right-click on the document and select Update Field
• You can also Select All, right-click and say Toggle Field Codes to see the recipes that Word uses to create these links

• You can right click on an individual Field, and select Edit Field... - this brings up this really powerful dialog:

  

• In File -> Options -> Display, you can tell Word to always Update fields before printing, to save you having to remember.