Plus = larger, semi-professional controller with hardware integration

MK4 = ultra-compact laptop controller.

maximum compact = MK4
Control hardware synths = Plus
Sequencer in the controller = Plus
modern software + affordable = MK4
more buttons = Plus

Both are not standalone, but are only midi controllers, which you can also use with the "Koala Sampler" app, for example (this is only a recommendation from me and is not included in the bundle). If you want something with internal sounds, then that would be the "Akai MPK Mini Play MK3" in this price and brand category. The differences: The "MPK Mini MK4" has fewer buttons, is smaller than the "MPK Mini Plus", but is more portable and has room in a rucksack. "Mini Plus" has 37 mini-keys (3 octaves) and "Mini MK4" has 25 mini-keys. Plus also has a joystick. Plus: OLED, MK4: colour display + encoder. Extended arpeggiator on the MK4. Plus has: Sequencer 2-Track Step Sequencer and CV/Gate (modular compatible). Plus also has a MIDI input. Plus: USB-B, MK4: USB-C.

Plus = significantly more hardware control

MK4 = more modern, but more minimal

The Plus has CV/gate, MIDI in/out and sequencer, for example, which means it can also be used with hardware synths or modular systems.

The MK4 is more of a compact DAW controller with an improved keybed and display.

Performance features
MPK Mini Plus

Additional performance tools:

Polyphonic 2-track sequencer
Chord Mode
Scale Mode
Arpeggiator
Joystick Controller
DAW Transport Buttons

This allows you to sequence ideas and control external devices even without a DAW.

MPK Mini MK4

New compared to MK3:
new keybed (Gen-3)
Pitch + Mod Wheels instead of joystick
Improved pads
colour display
new arp modes (pattern, freeze, mutate)

But:
no sequencer
no CV/Gate
less performance hardware

MPK Mini Plus
Typical scope of delivery:
MPC Beats
AIR Hype
AIR Electric
AIR Essential Keyboards
several drum and sample packs

MPK Mini MK4
Newer bundle:
Ableton Live Lite 12
Studio Instrument Collection (1000+ sounds)
Komplete Select 15 (with some bundles)
The MK4 software is more modern and comprehensive.

Practical difference
MPK Mini Plus is better suited for:
Hardware synth setup
Modular / CV
Sequencer-based jams
More key range (3 octaves)

MPK Mini MK4 is better suited for:
Laptop production
travelling / backpack
Ableton / FL Studio Controller
affordable entry