Multiple(4+) Macs to 1 Display

Hi, We are looking for a solution that allows us to share our individual screens to one display(mirrored). This would allow us to quickly discuss/critique projects on the fly to the whole studio and also to clients at meetings. We need to do this from a minimum of 4 Macs. Does anyone have any ideas?

Alternatively, a piece of software that allows all/individual user(s) to receive the screen data from a single Mac?

Many thanks in advance!
Tom.