How to use ifdefined inside align*

I need to use ifdefined inside align to tell latex to use different code for pdf and for tex4ht. But it gives an error.

Is there any way around this (other than putting the ifdefined outside everything, which will complicate things for me.

Here is MW

documentclass{article}
usepackage{amsmath}
usepackage{color}
usepackage{array}

begin{document}

begin{align*}    
R_{2} = R_{2}-R_{1} &Longrightarrowhspace{5pt}
ifdefinedHCode
   left[begin{array}{@{}ccc|c@{}}
                       0&0&4&0 
                       0&0&0&0 
                       0&0&2&0 
          end{array}
    right]               
else
     left[             
         begin{array}{@{}ccc!{color{red}vline width 0.6pt}c@{}}
                       0&0&4&0 
                       0&0&0&0 
                       0&0&2&0
         end{array}
     right]               
fi      
end{align*}
end{document}

Compiling with lualatex gives error

(/usr/local/texlive/2020/texmf-dist/tex/latex/microtype/mt-PalatinoLinotype.cfg) (/usr/local/texlive/2020/texmf-dist/tex/latex/microtype/mt-msa.cfg) (/usr/local/texlive/2020/texmf-dist/tex/latex/microtype/mt-msb.cfg))
! Incomplete ifx; all text was ignored after line 21.
<inserted text>
fi
<*> foo3.tex

?

TL 2020

documentclass{article}
usepackage{amsmath}
usepackage{color}
usepackage{array}

begin{document}

begin{align*}    
R_{2} = R_{2}-R_{1} &Longrightarrowhspace{5pt}
{ifdefinedHCode
   left[begin{array}{@{}ccc|c@{}}
                       0&0&4&0 
                       0&0&0&0 
                       0&0&2&0 
          end{array}
    right]               
else
     left[             
         begin{array}{@{}ccc!{color{red}vline width 0.6pt}c@{}}
                       0&0&4&0 
                       0&0&0&0 
                       0&0&2&0
         end{array}
     right]               
fi}     
end{align*}
end{document}

documentclass{article}
usepackage{amsmath}
usepackage{color}
usepackage{array}

begin{document}

begin{align*}    
R_{2} = R_{2}-R_{1} &Longrightarrowhspace{5pt}
left[
begin{array}{@{}ccc!{ifdefinedHCode |elsecolor{red}vline width 0.6ptfi}c@{}}
                       0&0&4&0 
                       0&0&0&0 
                       0&0&2&0 
          end{array}
    right]               
end{align*}
end{document}

documentclass{article}
usepackage{amsmath}
usepackage{color}
usepackage{array}
usepackage{etoolbox}

begin{document}

begin{align*}
R_{2} = R_{2}-R_{1} &Longrightarrowhspace{5pt}
ifdef{HCode}
   {left[begin{array}{@{}ccc|c@{}}
                       0&0&4&0
                       0&0&0&0
                       0&0&2&0
          end{array}
    right]}
   {left[
         begin{array}{@{}ccc!{color{red}vline width 0.6pt}c@{}}
                       0&0&4&0
                       0&0&0&0
                       0&0&2&0
         end{array}
     right]}
end{align*}
end{document}