I know it's possible bruteforce 4 missing words
Are you sure that this is possible? I know that for two electrum missing words, it can take around 20 seconds on an average pc. For three words it'll take 20*2048 = 40960 seconds which is equal with ~11.3 hours. But for 4 words... Oh boy. It'll take around 23,142 hours which is 964 days.
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 ?
Let's say that I don't know any of the words, but for 12 words (out of 13) I know the last one or two letters, and for two words I know the first letter.
I also know their order and even some of their length.
You can surely reduce it, by a lot. But still, brute forcing by not knowing 4 out of 12 words isn't meant to be found.
Well that's very true. Bruteforcing 4 words is completely waste of time unless we know a possible combination of words and the sequence in which those words might have been placed.
Otherwise it's completely a waster of time trying out so many combinations.