Square Up — ruling 1

Square Up overwrites all previous effects that set the target creature’s power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after Square Up resolves will overwrite this effect.

Square Up — ruling 2

Any effects that modify a creature’s power and/or toughness without setting them to a specific value (i.e. ones that don’t affect base power and/or toughness) will apply after its base power and toughness are set, regardless of the order those effects were created. The same is true for counters that modify its power and toughness.

Square Up : règles, fonctionnement et interactions (MTG)

Square Up is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target creature has base power and toughness 4/4 until end of turn.

How it works : Square Up

Original English text — Wizards of the Coast / Scryfall

#1April 16, 2021· Wizards of the Coast
Square Up overwrites all previous effects that set the target creature’s power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after Square Up resolves will overwrite this effect.
No other card is named in this ruling.
#2April 16, 2021· Wizards of the Coast
Any effects that modify a creature’s power and/or toughness without setting them to a specific value (i.e. ones that don’t affect base power and/or toughness) will apply after its base power and toughness are set, regardless of the order those effects were created. The same is true for counters that modify its power and toughness.
No other card is named in this ruling.