OP did not specify an use case which needs PCIE Gen4 or needing an overclocked 3900X for a gaming use case. At 1440p, money is better spent on the strongest GPU in the budget.

Also the Tomahawk has VRMs strong enough for an overclocked 3900X. See the AMD X570/B450 VRM tier list

The next better X570 covered in the VRM tier list is AUD70 more and this money, in my opinion, is better suited for the GPU for 1440p gaming.

As for 4th gen Ryzen, I do not have any information that specifically says that B450 boards, especially ones with power delivery circuitry as good as the Tomahawk, will not support at all. I am aware of Ryzen 3000 issues with the B450 Tomahawk but those have been mostly resolved with subsequent BIOS updates.