Hey,
@CrystalCat ! Long-time user of Holder's sprites here. I'm not affiliated with them, but I thought I'd give a try at helping you.
I understand what you're getting at. Unfortunately, you'll have to buy DragonBones if you want to use all four frames, or there may be a plugin that allows four frames to be used in the default MV/MZ format (I'm just guessing, there might be no such plugin). I see no problem in using just the last three frames, but that's because my animations are much less complex and look good with that setup. I actually would add a slight, "wind-down" effect on the last frame, just for a smooth transition. I know you said it can't have an effect, but the truth is, it actually can. In fact, it looks better that way.
To explain it in words, the third frame holds a larger, more dramatic effect, while in the fourth frame, the same effect is fading. BTW, each frame is 160x160, so when I'm making effects or even designing complex characters, I have a grid that helps me make what I like without going outside the lines.
Here's an example. This is a simple character for the purpose of demonstration, and I used a default weapon and edited it, so all credits go to Holder for the assets. I see the final frame as the "halfway point", and the effect appears to vanish when it returns to the "Idle" animation afterwards. I have yet to test this particular character, but I've done something similar before, so I'm confident it should work.
Give it a try and tell me how it goes. I'm willing to change my method if I have to, especially since if this doesn't work for you, it probably won't work for me at some point.