My goal was to design a network in a house (4 floors: basement, groundflor, first floor, second floor) with seamless roaming.

Hardware:
- 1 x RB3011 as a central router
- 4 hAP ac configured in bridge mode

hAP ac lite Details:
- 4 units named RB952-KG, RB952-EG, RB952-OG, RB952-DG
- configured in bridge mode (wlan1+wlan2+all ether ports) with dhcp client on that bridge
- valid for all hAPs: SSID on 2,4 Ghz: MOKRZY
- valid for all hAPs: SSID on 5 Ghz: MORKZY_5
- valid for all hAPs: same Wireless Key
- RB952-KG 2412Mhz/5180Mhz, bandwidth set to 20MHz
- RB952-EG 2437Mhz/5200Mhz, bandwidth set to 20MHz
- RB952-OG 2462Mhz/5220Mhz, bandwidth set to 20MHz
- RB952-DG 2412Mhz/5260Mhz, bandwidth set to 20MHz

Please be so kind and tell me if the setup is OK? Do I need to consider someting elese (b/g/n on 2GHz, only ac on 5Ghz)? Is it the best practice to choose same SSID for both frequencies? Am asking because my KindleFireTV seems to be having some issues with my 5GHz network. Ot seems like he is trying to connect to an accespoint with afairly week signal and keeps loosing that connection, swithich to another AP, and so back an forth.

Yes, the setup looks fine.

Choosing the same SSID is also regular practice. A different issue is devices struggling with certain frequencies, but most of the time (device OS allowing) it will connect to the better signal/SNR SSID.

Your scenario is ideal to setup a CapsMan deployment: you'll be able to forget about the APs, they'll be like "virtual" Aps inside the CAPsMAN controller (3011).

I would also set up a signal range for the devices in access list. otherwise phones might remain connected to a lower signal ap.

If it's a FireTV Stick that is having problems, mine only works channels 36-48. Maybe set the HAP next to it to a lower channel.