Submitting an archive, stefan.tar.gz which has your MWE slightly modified by adding two more bib items and multiple authors to check if everything works fine. Modified version of natbib.4ht is also included. If everything works fine, we will commit the changes to svn.

(file #126)

In the moment it is not possible to use tex4ht with citet, when using natbib with numbers and super.

Minimal examples are attached.

call:
-->
htlatex htlatex_test.tex
<--

Error:
-->
(c:/texlive/2011/texmf-dist/tex/generic/tex4ht/html4-math.4ht))
(./htlatex_test.aux)
! Missing } inserted.
<inserted text>
}
l.13 Test if citation \citet{test-1_1998}
works?
<--