documentclass[12pt,a4paper]{article}

usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{cfr-lm}         % instead of the "[cyr]{aeguill}"

usepackage[table]{xcolor}
usepackage{ragged2e}       % new
usepackage{booktabs,       % new
            makecell,       % new
            tabularx}       % new
renewcommandtheadfont{smallbfseries}
renewcommandtheadgape{}
newcolumntype{L}{>{RaggedRight}X} % redefined

%---------------------------------------------------------------%

begin{document}
    begin{table}[htb]
    footnotesize
    setlengthtabcolsep{4pt}
begin{tabularx}{linewidth}{@{}|
    >{large}c|
    >{hsize=0.25hsize}L|
    >{hsize=0.25hsize}L|
    >{hsize=0.50hsize}L|   @{}}
    hline
thead[l]{RG}
    &   thead[l]{Définition}
        &   thead[l]{Mapping}
            &   thead[l]{Règles de gestion spécifiques}                                               \
    hline
1   & Portail Date Traitement Courant
        & RBP vTBADMRB _SUIVI_APPLI.D _TRAIT
            &   CASE WHEN

                [RBP Presentation View].[Dimension Référentiel Datamart].[L_DATMR]

                = `PORTAIL' THEN [RBP  Presentation View].[Fait Suivi Application].[D_TRAIT] END  \

    hline
end{tabularx}
caption{Équipe de travail}
end{table}
end{document}