chore: v4.0.0 release

[rakita]

Manually run release-plz. Will add small refactorings to CHANGELOGs.

[github-actions]

Valgrind Results:

==3982== Cachegrind, a cache and branch-prediction profiler
==3982== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==3982== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==3982== Command: target/release/snailtracer
==3982== 
--3982-- warning: L3 cache found, using its data for the LL simulation.
Snailtracer Host+Interpreter benchmark (2.1s) ...            not enough samples
==3982== 
==3982== I   refs:      463,562,564
==3982== I1  misses:          3,451
==3982== LLi misses:          2,897
==3982== I1  miss rate:        0.00%
==3982== LLi miss rate:        0.00%
==3982== 
==3982== D   refs:      237,388,553  (160,565,089 rd   + 76,823,464 wr)
==3982== D1  misses:        323,608  (    183,803 rd   +    139,805 wr)
==3982== LLd misses:        137,542  (      4,062 rd   +    133,480 wr)
==3982== D1  miss rate:         0.1% (        0.1%     +        0.2%  )
==3982== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==3982== 
==3982== LL refs:           327,059  (    187,254 rd   +    139,805 wr)
==3982== LL misses:         140,439  (      6,959 rd   +    133,480 wr)
==3982== LL miss rate:          0.0% (        0.0%     +        0.2%  )