We have several RB4011 supplied by eth1/802.3at from switches of different brands (Cisco, HPE, Zyxel and others) without any problems. According to the switch logs, the RB4011 properly negotiates af/at on eth1, although not specified.
In our experience, the power consumption of a RB4011 with 4 or less ethernet ports in use and a 1GBit SFP copper module stays between 7W and 12W (depending on CPU load) and works reliable also when supplied from an 802.3af only 15W switch port.
If you use all Ethernet ports and especially with 10GBit SFP+ copper modules, it will go up to 18W. This is slightly above the at limit of 15W, but most switches also tolerate 18W load on a af port.
Another possibility to safe power is to use a DAC cable instead of Copper SFP to connect the RB4011 to your Cisco switch. The RB4011 is specified not to support DAC cables, but for short distances it works. We have 2 devices connected to Cisco switches using this 0.5m DAC cable:
https://www.fs.com/uk/products/40109.ht ... &id=222988
Both working without problems.
But its only working for short distances. With the 1m version of the cable above, the connection gets unreliable, with the 2m version no link is established at all.
But be reminded DAC cables are not officially supported, so you cannot expect any support from MikroTik for such setups. And I would not recommend it for client installations, just for your own sites.