If you absolutely want separate equipment here are my thoughts
I do not recommend turntables that are too entry-level such as the at-LP140XP, you will quickly see their limits for DJ use, you will be disappointed with your experience (lack of motor torque/chassis too sensitive to rumble) and you will want to resell them. You will end up losing a lot of money on this type of equipment. For vinyl turntables, it is better to save a little longer, and choose a model that will give you complete satisfaction over time and that you will keep for years.
If I were in your situation today I would certainly consider some Reloop RP 7000 MK2 which is probably the best quality/price ratio on the market. Of course this will require a higher initial investment, but there is little chance that you will want to resell them and therefore lose money in the long term.
Concerning the mixer, even if I find the x1850 very cool, it is too fragile for me. It’s the lottery, either you will come across a unit with which you will never have problems, or you risk problems on the faders as well as on the OLED screens which will fail for no reason overnight. I know several people who have had quality issues with their x1850 so I couldn’t recommend a mixer as great as it is on paper which seems to have so many quality control issues.
If you absolutely want a 4-channel mixer with a reasonable price, I think the Reloop RMX90 DVS or a Pioneer DJM 750 MK2 are reliable alternatives to consider. Be careful, however, the DJM 750 Mk2 is not serato compatible unlike the RMX 90 DVS.
You could save quite a bit of money if you started with a small 2-way battle mixer such as a Numark Scratch for example which unlocks the original Serato. Which with RP 7000 MK2 gives you a serato compatible DVS pack for around €1500, a little more expensive than an SC Live 4.
But this will obviously be a much less mobile configuration, so you must also take this point into account in your needs
If your goal is mobility, turntables are probably not the best choice. If the vinyl feel is not an absolute necessity, a controller or a standalone product such as the SC Live 4 is certainly more suitable. And if you’re looking for mobility, vinyl feeling, 4 channels and phono inputs to connect vinyl turntables, then probably the Rane Performer is the way to go.