What should I buy?

Your online community profile shows you already have the Arc Ultra with built-in SoundMotion (Sub) - so I’d first go with the Era 100’s for surround sound and then add the Sub afterwards, as the SoundMotion unit on the Arc-Ultra is pretty good already for its size and purpose. Things though do improve a lot more when a full size Sub is added to the HT setup.

Yes, I have an Arc Ultra, I want to give it more cinematic feeling, do you recommend me to buy two Era 100 and then the big Sub Gen 4?

Yes, that sounds like the best way forward to me if all are within your chosen budget.

The problem is that there is no space to put the two Era 100 nor the two Era 300, neither behind, nor to the sides, that's why I was going to start with Sub gen 4, and the Era 100 I can't put them behind the TV84.

If you can’t put two speakers behind you, or to the side of your listening position, which you’ve not previously mentioned, then it’s a matter for you what you get - the two Era’s though, when bonded into a Sonos Home Theatre setup are for rear (& pseudo-height) channel TV audio - so they're certainly not intended to be used as front channel speakers when watching TV with surround sound. You would likely ruin the front channel audio in some cases, if you do intend to do that.