One well upgraded and equiped agent can be more effective sometimes than 4 poorly equiped agents. It allows you to progress in the early game without risking ill prepared yet valuable agents.
I've often played with just one or two agents for this very reason. I prefer to use one or two fully upgraded agent (more durable) to four poorly upgraded agents. Off course some missions lend themselves better for this playing style than others. Making good use of 'panic mode' and of the persuadertron also helps. I've managed to beat a lot of levels using a mob of persuaded people and just one agent. One agent can also do a ton of damage with a couple of lasers, miniguns and an upgraded brain.
For those who haven't playes the game: if you activate panic mode (press both mouse buttons at once) your agent will move faster and will automatically fire at threats. The higher the level of the brain is that your agent has, the longer you can use panic mode and the longer the range at which he will automatically fire.