I don't understand so how much current it can drive then ?
if you say it has impedance of 455 ohms at 4.6v its ~10 mA ?
what I learned in school is look at what the data sheet say and don't try to interpret it in different ways .
"Datasheet says" works fine when you're at or below the numbers. Example, if you were asking about < 0.88mA, you can be guaranteed to have 0 to 0.4V or 4.6 to 5.0V as output levels.
When you're asking about more, you have to make assumptions about the physics of the device. If they provide a typical output V-I curve, you can use that to infer what happens at other voltages. You aren't guaranteed to have those curves met (the guaranteed data is only a single point on the graph), and in principle, the curve could be anywhere else on the graph, as long as it meets the limit in that small range where the data says it has to be.
Note that you may have to view the logic family datasheet to find these curves.
In reality, there is absolutely no physically possible reason for a simple output stage to exhibit anything more complicated than the curve given. The variable that they're protecting against is a matter of slope and scale; some devices will roll off sooner than others, some will be steeper or shallower than others (higher or lower resistance in the Rds(on) region).
Although CMOS outputs do flatten out at large voltage drops (tending towards a constant current characteristic), it's not usually much, and you can guess the short circuit current Isc = Vdd / Rds(on) is a good guess. It might be as much as double the actual value, but this is well into the extrapolated range, so that shouldn't be unexpected.
The most important consequence is this: you aren't going to get more power output, or drive capability, than Vdd^2 / Rds(on) suggests. And more likely, a quarter of that, because of the power matching theorem (i.e., Rsrc = Rload, which isn't a terrible approximation for the known output V-I curves here).
So if that's not enough drive power for your load, you'll need something better.
Have you considered a MOSFET?
Tim