FOUR and TEN are both using the same Soundster: surFING.

Both are differentiated by using different tones for “ser”.

FOUR uses the high-low tone for “ser”. Check out the sound here.

TEN uses the low-high tone for “ser”. Check out the sound here.

Use additional signs like raising four fingers as you say “ser” to give additional cue to your listener that you are saying FOUR.

And of course you can raise all ten fingers when you are saying “ser” for TEN.