definitely a cheat option, but I feel like it could be worthwhile to add options to adjust the stats (energy, top speed, and acceleration) in case someone (me) wants to make the battery pack 4 more of an upgrade than a sidegrade

These options already exist. Please go to Settings → Mod settings → Startup → Battery-Electric Train and check out the available options. All options have explanations behind the (i) icons.

All of the options influence all battery packs equally, though, so it will not be possible to make pack 4 an upgrade of pack 3. This is intentional because all properties of the electric locomotive and the battery packs scale proportional to the base game. Pack 3 gives you more capacity/range and pack 4 more acceleration, because that’s the same trade-off you have to make when choosing between filling your combustion locomotives with rocket fuel (capacity/range) or nuclear fuel (acceleration). Also have a look at this mod’s FAQ, where I explained that in a bit more detail.

fair enough, it's just a bit confusing (for me) to have the "battery pack 4" not be a direct upgrade to the "battery pack 3" when that's been the pattern up until then

You’re not too far off there. The battery pack 4 used to be a direct upgrade to the battery pack 3. This mod was originally made for Factorio 1.1, in which the nuclear fuel was a definite upgrade compared to rocket fuel because the nuclear fuel’s stack size used to be 10, resulting in more total energy compared to a full stack of rocket fuel. In Factorio 2.0, the nuclear fuel’s stack size was changed to 1, so nuclear fuel is not an upgrade to rocket fuel anymore when it comes to total stack energy. This doesn’t look that nice when it comes to the four numbered tiers of battery packs, but I couldn’t come up with anything better, so the names stayed the same, despite the battery pack 4 getting nerfed.