Here is a solution redefining \textcite
.
\documentclass{article}
\usepackage{filecontents}
\usepackage[style=authoryear,natbib]{biblatex}
\addbibresource{\jobname.bib}
\begin{filecontents}{\jobname.bib}
@article{one,
author = "Last, First",
title = {Title},
journal = {A Journal},
year = {2012},
}
@article{two,
author = "Last, First",
title = {Title},
journal = {B Journal},
year = {2012},
}
@article{three,
author = "Surname, Given",
title = {Title},
journal = {C Journal},
year = {2013},
}
@article{four,
author = "Surname, Given",
title = {Title},
Journal = {D Journal},
year = {2013},
}
\end{filecontents}
\makeatletter
\DeclareCiteCommand{\textcite}
{}
{\iffieldequals{namehash}{\cbx@lasthash}
{\iffieldequals{year}{\cbx@lastyear}
{\addcomma\printfield{extrayear}}
{\addcomma\addspace\printfield{labelyear}\printfield{extrayear}}%
\ifnumequal{\value{citecount}}{\value{citetotal}}
{\bibcloseparen}
{}%
}
{\ifnumequal{\value{citecount}}{1}
{}
{\bibcloseparen\multicitedelim\addspace}%
\printnames{labelname}
\bibopenparen\printfield{labelyear}\printfield{extrayear}%
}%
\savefield{namehash}{\cbx@lasthash}%
\savefield{year}{\cbx@lastyear}%
}
{}
{}
\makeatother
\begin{document}
\citet{one,two,three,four}
\printbibliography
\end{document}
Producing

EDIT corrected problem with same author different year (missing comma and space between years)