documentclass{article}
usepackage{showframe}
renewcommandthetable{Roman{table}}

usepackage{tocloft}
renewcommandcfttabindent{0pt}% <-
renewcommandcfttabnumwidth{5em}
renewcommandcfttabpresnum{tablename }
renewcommandcfttabaftersnum{:}

begin{document}
listoftables
begin{table}[htb]
caption{First table}
caption{Second table}
caption{Third table}
caption{Fourth table}
end{table}
end{document}

documentclass{article}
usepackage{showframe}
renewcommandthetable{Roman{table}}

usepackage{tocbasic}
DeclareTOCStyleEntry[
  indent=0pt,% <-
  dynnumwidth,
  entrynumberformat=entryprefix{tablename}
]{tocline}{table}
newcommand*entryprefix[2]{#1 #2:hfill}

begin{document}
listoftables
begin{table}[htb]
caption{First table}
caption{Second table}
caption{Third table}
caption{Fourth table}
end{table}
end{document}

makeatletter
renewcommand*l@table{@dottedtocline{1}{0pt}{2.3em}}
makeatother