SuttaCentral
Video here explains the difference.
“That’s not ekaggata, THIS is ekaggata”
In a more general context, yes, first jhana can be described as ekaggata. But in the gradual samadhi in the 4 jhanas formula, 2nd jhana is distinctly more powerful, and as one is first learning the jhanas, 2nd jhana can be exponentially stronger, felt in the piti-sukha, as the similes in AN 5.28 aptly describe.
But then the third jhana and fourth jhana samadhi are even stronger
On the other hand, some say the progression from 1st thru 2nd and 3rd can be seen as relatively subtle gradation, but the transition to the 4th is more drastic, and difficult to attain – similar to the marked change upon attaining the 1st jhana.
Agree then what is it?
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If there is no Pithi and Sukha in the fourth Jhana what happiness you have with Ekagata?
https://dhammawheel.com/viewtopic.php?f=13&t=32940&p=493770#p493770
Interesting Hindu video.
ek’-agga citta means the mind has a single preoccupation. it’s not jumping around randomly.
see the entries for ekagga and ekodi-bhava for details
The question is why there is no Pithi and Sukaha.
That’s just the nature of the fourth jhana. It wasn’t concocted to meet our desires. 
I understand it.
But what sort of mental state you have in Forth JHana?
In purified jhana the mind is like water being purified from sand particles. Each vat/jhana purifies it some more:
To mix the metaphor going from hindrance free samadhi to the first jhana is like water suddenly freezing and turning into ice. The force of samadhi would be the binding force of the ice compared to water and this builds up to the fourth jhana.
The fourth jhana is like the ice made up of purified water. It has awareness and equanimity.