MediaWiki 1.4 is probably the first wiki I ever edited, so it's clearly
the most perfect version ever produced. To be honest, there were some
improvements that you may miss:
<https://www.mediawiki.org/wiki/Manual:Upgrading#Why_upgrade.3F> (I just
added one bullet).
I'm not sure what makes you so confident that 1.4 had no security
vulnerabilities: if you have a method to produce such statements, I'd
love to know it.
<https://www.mediawiki.org/wiki/Release_notes/1.5#MediaWiki_1.5.1>
reports some security fixes and there is no mention of them being
introduced in 1.5.0. (Private wiki doesn't mean it's bound to localhost
or an intranet, I suppose.)
I've recently heard of at least one person upgrading MediaWiki from 1.8
to 1.23, if I'm not mistaken; provided that you pay particular attention
to the upgrade notices for 1.5–8, there are good chances that the
upgrade goes well, though we're not currently able to swear it. In the
long run it's hopefully less effort than trying to run ancient PHP on
Debian.
Nemo