1. He observes keenly, but says little.
2. He watches keenly, but says little.
3. He looks keenly, but says little.
4. He notices keenly, but says little.
(Are they all the same? What is the similar sentence to Sentence 1?)
No there not all the same, the verbs are a bit different.
The similar sentence to 1) is 2)because observing is watching's synonym.
All the verbs are very similar, but with only slight nuances in meaning.
1. Yes, all of these sentences convey a similar meaning. They all describe someone who is observant or attentive but tends to speak very little.
The similar sentence to Sentence 1 would be Sentence 2: "He watches keenly, but says little." Both sentences use the verb "observe" and "watch" respectively to convey the idea of paying close attention, and they both mention that the person speaks little or says very little.