Messi was suspended for insulting the linesman during his national team's 1-0 World Cup qualifying win over Chile last week.
The ban was announced just hours before La Albiceleste's game away to Bolivia on Tuesday.
In the absence of their talisman, Edgardo Bauza's team lost 2-0 in La Paz, leaving them fifth in CONMEBOL qualifying and facing the prospect of needing to win an inter-confederation play-off if they are to reach next year's finals in Russia.
Messi's Barca team-mate Gerard Pique, speaking late on Tuesday, condemned the severity of the suspension and the club have now publicly issued a message of support for the 29-year-old forward.
"FC Barcelona expresses its surprise and outrage at the actions of the Office of Disciplinary Committee of FIFA to sanction the player Leo Messi after the qualifying match of the FIFA World Cup between Argentina and Chile," a statement read.
"The club considers the sanction of four games imposed against Messi unfair and totally disproportionate.
"Finally, FC Barcelona wishes to reiterate its support for Leo Messi, an exemplary athlete, for his behaviour on and off the pitch."