I am no expert, and there are far more knowledgeable people on here than I. That said, I believe that A. your audio shop "staff" is clearly trying to profit from you selling a $1000 plus AVR that you absolutely DO NOT need (based on what your use is) and B. revert to A. I copied and pasted this from AVS, maybe this will help:
Denon AVR-X4100W ($1,399 msrp)
The new AVR-X4100W is basically an X4000 with Atmos, HDMI 2.0, Bluetooth, WiFi, and an extra HDMI input. There are some other minor changes (noted below) but these are the key differences most will care about. In order to support the variety of potential speaker options, the pre-out count has been upped to 13.2 outputs (from 11.2 on the X4000), although note that a maximum of 9 channels can be active at a time regardless of how many external amps you add. The additional pre-outs are for flexibility if you want to have the option of switching between various speaker configurations (e.g. 7.1 + Wides for DTS Neo:X, and 7.1 + In-ceiling speakers for Dolby Atmos).
Denon AVR-X5200W ($1,999 msrp)
The X5200W is basically a Japan-made, souped up X4100W. It adds two more internal amps (9 total) rated at 140W/ch into 8 ohms (up from 125W/ch of the X4100), two more channels of processing capability (can run 11 simultaneously instead of 9 max), and fancier gold plated speaker connectors. Like the X4100W, the X5200W has a full set of 13.2 channel pre-outs, but since it can run 11 channels simultaneously, you can switch on the fly (example) from 11ch Neo:X and 7.1.4 Atmos, provided you have sufficient channels of external amplification to supplement the 9 internal amps. The X5200W also borrows from the higher end 4520CI (and its replacement X7200W) by incoporating a beefier "monolithic discrete amplifier" design with upgraded "Denon High Current Transistors" (DHCT), so it can be assumed it will be much more capabable of handling difficult speaker loads than the lower level models.
Hope this helps.