R12 (Z490 chipset) has M2. slot for NVMe ssd on mobo. One M.2 2230/2280 solid-state drive PCIe NVMe up to 32 Gbps Up to 2 TB [URL] R9 (Z370) too. One M.2 2242/2260/2280 solid-state drive [IMAGE] Aurora R9 For your need of two X4 slots and M.2 in Dell pc, you could also consider an older system with three x 4/8/16 slots and no M.2 (if such Dell system exists) and use a PCIe adapter to install M.2 in one of the x4 slot.  
My limited understanding of why there's a shortage of x4 slots in the newer XPS units I have is that it's due some chipset only allowing so many lanes in total.   For example, the 8930 has... Then the much older 8900 has... My guess is that the on-board M.2 NVME slot in the 8930 consumed extra lanes and thus Slots 2 & 3 could only support x1.   Are there any Dell systems that somehow support dual x4 slots in addition to an on-board NVME slot?
XPS - lanes & x4 slots