Output current capability is the maximum usable output current the device can source while still maintaining the specified output voltage. As long as your load requires less than 300mA, the device will operate.
So in your example with 4 IC's drawing 100mA each, that is 400mA total, which is above the 300mA specified output current, so this will not work.
Connecting to two IC's would be a total of 200mA, which is below the specified capability of 300mA, so everything should be fine.
Normally the specifications have a little leeway so that if you had 3 IC's drawing a total of 300mA, the regulator should still work, but I would not suggest running this close to the limit....maybe on a bread board test circuit, but never in something going to production builds...history says something that close to the limit will come back and bite you at some point.