Bipartite Graphs and Matchings · Easy problem

A network has two hub stations $A$ and $B$ directly linked to each other. $A$ is also linked to devices $1$ and $2$; $B$ is also linked to devices $3$ and $4$. Activating a link uses up both stations or devices it touches, so no station or device may be in more than one active link. What is the maximum number of links that can be active at once?