First thing people need to understand: this whole world operates at scale. You don't bother making an ASIC unless you really need one -- you're either making millions/hundreds of millions of them, or the individual application is so high-value that the hassle, complexity, and expense of moving off a general-purpose CPU onto something completely custom is worth it.
Second, 4K is chump change. You need an entire team of people to do anything meaningful with FPGAs/ASICs. Less than that, just buy a $100 CPU that will do more, be immediately available, and have an entire software stack (OS, user-space programs, compilers, etc) for it. And teams that know how to build this stuff well cost MONEY. A lot of it. Not $100K/year. More like $250-300K minimum per person at Silicon Valley rates.
I get that the OP wants to do this as a sort of hobbyist undertaking, just realize, he's cutting against the grain of the entire ecosystem. Everything about this stuff is made for huge companies to do massive projects that will ship at gargantuan scale. Not hobbyists in a garage.
The major part of hardware development is NOT tooling. It is manufacturing the hardware.
You are the first person in this thread about FPGAs to mention making an ASIC.
They're really, really not.
Linting tools, simulation tools, formal verification and synthesis tools are all the same. Verification methodology is also the same.
However, for open source to flourish, high quality open source simulators are required. I think if we had that, synthesis and place&route would follow. You don't need synthesis or par to do design and verification, but you do need simulation.
I look into the state of OS simulators periodically. Some are impressive, but mostly just handle the design basics... an OS systemverilog, static and dynamic, simulator would be a game changer.
It's a Herculean task though. I don't know how much Xilinx makes off tooling. I have to think they would make more from FPGA sales if tooling were free, and if it were open source, it would no doubt be improved upon by degrees I can only fantasize about.
Simulators, and by extension synthesis and place & route tools, are complementary goods to FPGA hardware. Make your complementary goods cheap, and you make a lot of money.
But there are tool companies out there such as Mentor and Synopsys that make a lot of money from such proprietary tooling. I imagine simulators have a tough patent field to navigate.