Fix: List a max of four share owners instead of three

[SamuXarick]

Motivation / Problem

An AI company may have four different share owners, but the company information window is limited to display a max of three.

Description

This commit increases that limit.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[nielsmh]

LGTM

[glx22]

How is it possible for a company to not own at least 25% of its shares ?
I think the issue is not the display, but in the share side, a company should always own at least 25% of itself, else it's merged into another one.

[2TallTyler]

A real-world example of a company being completely owned by other companies would be terminal railways in the US, which are jointly owned by several railroads that all need access to a major station, railyard, etc.