The bandwidth used by the PS4 for Twitch streaming for the Best, High, Medium, and Low settings are 1500kbps, 1200kbps, 860kbps, and 520kbps respectively. Given these figures, the highest you can use with a 1Mbps limit would be Medium, and that's also assuming there aren't any other people/devices other than your PS4 sucking much of that limit.
The biggest difference between the speeds are image resolution and sound quality; Best and High both broadcast at 960x540 image resolution and use 64kbps audio whereas Medium and Low settings broadcast at 640x360 resolution and use 32kbps sound. Unfortunately for your viewers there is a very big and noticeable drop in audio quality between 64 and 32kbps. The drop in resolution is noticeable but isn't intolerable unless someone is so used to watching everything in HD that they're too much of a quality snob to accept anything less. All four settings broadcast at 29.97fps.
For a basis of comparison, even the highest PS4 setting is only about half the upload bandwidth that a lot of PC streamers on Twitch use. In order to get a viewer's idea of what your stream may look like, your best option might be to watch some other PS4 streams and ask the users what quality setting they broadcast on until you find someone else using Medium.
Source: Eurogamer.net tech analysis article - the article contains a table with the numeric data detailed above, as well as side-by-side visual comparisons of the different quality settings when they tested them using Assassins Creed 4 on a Twitch stream.