\documentclass[a4paper, 12pt]{memoir}
\usepackage{fontspec}%			%% Use of the system-wide installed fonts is enabled
%\fontspec{libertinusserif-regular.otf}%	%% Default-font
\usepackage{amsmath}%			%% For using mathematical symbols
\usepackage{amssymb,amsfonts,textcomp}%	%% Special mathematical characters
\usepackage{lettrine}%			%% Fancy first letters of paremias
\usepackage{xifthen}%			%% Logic, logic, and logic, what else is there without logic!
\newif\ifEmptyPar%			%% false = Filled parameter, true = empty parameter; set by calling \MTPPR {eMPTyPaRameter} ==> is the inverse of \ifFilledPar
\newif\ifFilledPar%			%% true = Filled parameter, false = empty parameter; set by calling \MPTPR {eMPTyPaRameter} ==> is the inverse of \ifEmptyPar
\makeatletter				%% Starts the makeat-environment
\newcommand\MPTPR[1]{\@ifmtarg{#1}{\EmptyPartrue \FilledParfalse}{\EmptyParfalse \FilledPartrue}}%
\makeatother				%% Stops the makeat-environment
\newcommand{\MAINTEXTBODY}[3]{\MPTPR{#1}%
			      \ifEmptyPar Normal call of the package lettrine so 4 lines are used\par
					  \lettrine{#2}{}#3\par%		%% Normal text-body so full height for the Initial {#5}
			      \else Adjusted call of the package lettrine so #1 lines are used\par
			            \lettrine[lines=#1]{#2}{}#3\par%	%% Due to page-constraints we change the height of the Initial {#5}
			      \fi%
			     }%
\newcommand{\BLATEXT}{ext without the starting initial bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla.}%
\begin{document}
\setlength{\parindent}{0pt}
%\renewcommand{\LettrineFontHook}{\fontspec{libertinusserifinitials-regular.otf}}
%\fontspec{libertinusserif-regular.otf}%
%\setmainfont{libertinusserif-regular.otf}[ BoldFont = libertinusserif-bold.otf,
%					   ItalicFont = libertinusserif-italic.otf,
%					   BoldItalicFont = libertinusserif-bolditalic.otf
%					 ]
\setcounter{DefaultLines}{4}%
\renewcommand*{\DefaultLraise}{0.075}%
TESTING the macro

\MAINTEXTBODY{}{T}{\BLATEXT}

\MAINTEXTBODY{2}{T}{\BLATEXT}

\bigskip

VERYFING the macro

\lettrine{T}{}\BLATEXT

\lettrine[lines=2]{T}{}\BLATEXT%
\end{document}
