How do I use /testfor to help for a Capture The Flag game?

This command makes it so if there are less then 4 players and there within a radius of 5 then it will display an error message.

/execute if entity @a[limit=..4,distance=..5] run say Not Enough Players

Impulse Command Block (Power by a pressure plate at the spawn point):

/tag @p add w

Repeating Command Block:

/execute @a[c=4,r=10] ~ ~ ~ tag @a remove w

Repeating Command Block (Delay in ticks set to 100):

/w @a[tag=w] Not enough players, there must be 4 players to start!

Try this:

/tag @a add sel
/tag @a add sel
/tag @a add sel
/tag @a add sel
/say 4 players
/tag @a remove sel

Add commands before the last command (the remove command should be last in the chain). Everything should be a chain conditional, except for the first one which should be an impulse unconditional (assuming that’s the start). The very last command block should also be unconditional.