Ooops... sorry for the problem in my patch.

I had overlooked it because when you do the 1st pass w/o local tex4ht.sty it works. The problem is caused by subsequent \def\next while \next has been \let to \iffalse.

Attached is another patch tex4ht-sty-1.diff achieving the early \closein where I fixed that by \def-ining \next instead of \let-ing it.

Well, this one is not really fixing anything, just beautifying the code IMHO. At least this patch has the merits of explaining why my 1st patch did not work.

(file #281)

Hello,
When one compiles tex4ht-sty.tex, this file checks the presence of tex4ht.sty, and if present it uses it, otherwise it uses an inlined replacement of it.

The problem is that it does not test for the local tex4ht.sty, but for any tex4ht.sty in the TEXMF. This has two bad consequences :

1) If I have a tex4ht.sty installed in my LaTeX distribution, then this one will be used instead of the local replacement. So if the installed tex4ht.sty is not forward compatible with the current needs of tex4ht-sty.tex that won't work

2) Even if the installed tex4ht.sty works, this is problematic, because this means that the local inlined replacement is never used/tested.

I attached a fix that works for me. The fix is very simple, instead of:
I have now (with current dir):
On top of that I did some other change so that the \closein15 comes as soon as possible. I did this change because it is useless to keep this input file open all along, as the purpose of opening it was just to make some \IfFileExists test in plain TeX, so one can, and IMHO should, close it as soon as this objective is reached.