I have a ThinkPad T440 that came with the new adapter plug "slim-fit". The original charger is 19V, 3.42A. I ordered a replacement AC for travel, and Lenovo sent to me a charger that is 20V, 4.5A, with the same polarity. I'd want to ensure that it is totally safe the use the new one with higher voltage and current rating. From what I have read on various forums, they say it is OK to use the new one. Is it safe to go ahead and start using the 20V, 4.5A one?
Yes you can use it. At such voltages an 1V difference isn't much (it would be dangerous at a lower voltage though, an extreme example would be a nominal voltage of 1V and you give it 2V instead, in this case it's the double and can very well damage the device).
I've personally used a Thinkpad T60 on 26V instead of 19V and never had any issues, same with my current Dell E6400, so an 1V difference is nothing compared to that.
We got about 10 of these same adapters around our office.
They are used daily with T440 and X240 without issue.
The extra current is perfectly fine. The circuitry inside the laptop determines how much current is drawn. And it will only draw as much as it needs. It only means that the adapters maximum capacity will not be fully utilized.
The voltage difference is well with the variation acceptable to the laptop.
Typically the voltage may vary from 5% to low up to 10% to high.
20/19 = 1.0526
Slighty over 5% is nothing to worry about.