How to get the same amount of space before each arrow?

You have several options. I use the following helper macro throughout this:

newcommandmeta[1]{$langle$#1$rangle$}

We have to put relax after if the following line starts with [, or else this is interpreted as the optional argument to .

tabular

Perhaps the easiest.

begin{tabular}{ll@{ $rightarrow$ }l}
  [4] & meta{Instruction} & meta{Assign} relax
  [5] &                    & meta{If} relax
  [6] &                    & meta{While} relax
  [7] &                    & meta{Print} relax
  [8] &                    & meta{Read} relax
  [9] & meta{Assign}      & [VarName] := meta{ExprArith}
end{tabular}

tabbing

Just because this is seldomly used.

begin{tabbing}
  [4] = meta{Instruction} = $rightarrow$ meta{Assign} relax
  [5] >                    > $rightarrow$ meta{If} relax
  [6] >                    > $rightarrow$ meta{While} relax
  [7] >                    > $rightarrow$ meta{Print} relax
  [8] >                    > $rightarrow$ meta{Read} relax
  [9] > meta{Assign}      > $rightarrow$ [VarName] := meta{ExprArith}
end{tabbing}

---

Complete example showing tabular and tabbing:

documentclass[]{article}

usepackage{array}
newcommandmeta[1]{$langle$#1$rangle$}

begin{document}
begin{tabular}{ll@{ $rightarrow$ }l}
  [4] & meta{Instruction} & meta{Assign} relax
  [5] &                    & meta{If} relax
  [6] &                    & meta{While} relax
  [7] &                    & meta{Print} relax
  [8] &                    & meta{Read} relax
  [9] & meta{Assign}      & [VarName] := meta{ExprArith}
end{tabular}

begin{tabbing}
  [4] = meta{Instruction} = $rightarrow$ meta{Assign} relax
  [5] >                    > $rightarrow$ meta{If} relax
  [6] >                    > $rightarrow$ meta{While} relax
  [7] >                    > $rightarrow$ meta{Print} relax
  [8] >                    > $rightarrow$ meta{Read} relax
  [9] > meta{Assign}      > $rightarrow$ [VarName] := meta{ExprArith}
end{tabbing}
end{document}

Result:

---

Other options

Instead of tabular you could use longtable to get something that is page breakable.

description environment with enumitem

This one is admittedly a bit of a hack, but one could also define a formatting macro and use a description list. The following defines myformatter to format the start of each row. It takes two optional arguments, the first should be the widest entry of the second column, in this case Instruction, the second should be the widest number in the line numbering.

documentclass[]{article}

newcommandmeta[1]{$langle$#1$rangle$}
newcounter{mylinenumber}
makeatletter
NewDocumentCommandmyformatter { O{} O{0} m }
  {%
    [phantom{#2}llap{arabic{mylinenumber}}]% right aligned, brackets outside
    %[rlap{arabic{mylinenumber}}phantom{#2}]% left aligned, brackets outside
    %phantom{[#2]}llap{[arabic{mylinenumber}]}% right aligned, brackets packed
    %rlap{[arabic{mylinenumber}]}phantom{[#2]}% left aligned, brackets packed
    hspace{1em}%
    ifx@itemlabel#3%
    else
      rlap{meta{#3}}%
    fi
    phantom{meta{#1}}%
    hspace{.5em}$rightarrow$hspace{.5em}%
    stepcounter{mylinenumber}%
    ignorespaces
  }
makeatother  

usepackage{enumitem}

begin{document}

setcounter{mylinenumber}{6}% number of the first row
begin{description}[{format=normalfontmyformatter[Instruction][10]}]
  item[Instruction] meta{Assign}
  item meta{If}
  item meta{While}
  item meta{Print}
  item meta{Read}
  item[Assign] [VarName] := meta{ExprArith}
end{description}
end{document}

Result:

There are different ways how you could align the brackets and line numbers, the one shown above is the one not commented out. Pick the version you like best by commenting out the others while keeping the one alignment you like (the comments after each line should hint at what this looks like).

---

Adding the line numbers automatically in the tabular approach.

documentclass[]{article}

usepackage{array}
newcommandmeta[1]{$langle$#1$rangle$}
newcounter{mylinenumber}
newcommand*mylinenumber[1][0]
  {%
    [phantom{#1}llap{arabic{mylinenumber}}]%
    stepcounter{mylinenumber}%
  }

begin{document}
setcounter{mylinenumber}{4}% number of the first row
begin{tabular}{>{mylinenumberhspace{1em}}l@{ $rightarrow$ }l}
  meta{Instruction} & meta{Assign} relax
                     & meta{If} relax
                     & meta{While} relax
                     & meta{Print} relax
                     & meta{Read} relax
  meta{Assign}      & [VarName] := meta{ExprArith}
end{tabular}
end{document}