Yes, you are correct, in that the Draytek has full control & you can use its firewall etc to protect your network, even when connecting through the 4G router.
You can download the User Guides etc here: Draytek Vigor 2860 downloads
Actually - Draytek also do their own dedicated 4G router in the 2860 range: Draytek Vigor 2860Ln
For me, the 2860 Vac I think is a bit more flexible, as it also has built-in ADSL/VDSL functionality, you just have to add your own 4G connection (via a 4G router connected to the Draytek via the WAN2 ethernet port, or a 4G USB "dongle" in one of the Draytek's USB ports).
The Draytek Vigor 2860Vac has 4 WANs - WAN1 is provided by the built-in ADSL2/VDSL2 modem, so you can use that for ADSL or FTTC. WAN2 is provided by an ethernet port connection - this is what I currently use for my 4G router - but because the router is in an upstairs window (to obtain best reception - the higher the better is usually true for 4G routers & antennae), and the Draytek is downstairs near my master socket, I connect the 2 via ethernet cables & powerline adaptors (similar to these ones [although mine is an older, cheaper & slower version]: TP-Link Powerline Adapters).
WAN3 & WAN4 are the USB ports, as you can use 3G/4G "dongles" for a 4G connection (like this one: Hauwei E3372 4G Dongle). In fact, these dongles are another option, especially if they can accept an external antenna (the E3372 can).
If I eventually get either a Fixed Wireless connection, or an FTTP connection, then that would have to connect via the WAN2 port instead of the 4G router (which would still be useful as a backup - or I could add a 4G USB "dongle" to one of the Draytek's USB ports, plus either a small antenna (e.g. Small 4G Antenna), or you can even connect the larger Poynting XPOL antenna to a 4G dongle via a CRC9-to-SMA adaptor (e.g. SMA-to-CRC9 adaptor)).
So, in summary: I use the Draytek to "load balance" my ADSL connection (WAN1) with my 4G connection (WAN2), but the Draytek provides protection & control for the combined ADSL + 4G link.
The Draytek actually provides a lot more control & options than I have had a chance to explore yet.