Whenever I am in such a situation where I have to perform the loops faster I do the below thing. Let me know if it can work well in this scenario as well.
So let's consider that 4 words will take 964 days according to you. So when we write a program to bruteforce the seed one program will take 964 days.
Now let's restrict that program to limited amount of combination and create another program with another set of combinations such that both programs have different set of combinations and are executed in parallel.
In this way both will be executed simultaneously and we will get the results in half of it's time meaning 964/2 = 482 days.
If we repeat this and create 10 such programs then the speed of results will be 10 times faster. Even if we don't run it in the same machine, we can run them in different machines.
So 964/10 = 96.4 days.
How do you find the above approach ?
what you are describing is the definition of parallelism and is used by any modern CPU that has more than one core. you don't need to write or run different programs, you just write once but split the workload between multiple CPU cores and each core performs the same task on a different data but separately and in "parallel".
CPUs can have between 2 to 64 cores.
GPUs have hundreds of cores and can run a lot more operations in parallel which is why using GPU in brute forcing is very common and the speed gain is huge.
with that in mind i don't think it is hard to recover a seed with missing 4 words.