This blue team ability allows the player to put back in the blip pile a number of Genestealers equal to the result of a die roll.

If the player rolls for example a 4, can he choose to send back only 3 Genestealers (in hope that the fourth one will be delt with in a more definitive way) ?

To me, using the ability is optional, but the number shuffled back isn't. There is no "up to" in the text, nor does it really feel thematic for the Marines to intentionally leave enemies within range. Also note that you can only shuffle back GS engaged with the Blue team's Marines.

Of course, an empty formation is always better than a formation with 1GS in it, at least in my book.

well the thematic issue makes sense.( I have a french copy of the game, and I wasn't sure of the way to understand the instructions about either choosing the number, or choosing the genestealer cards returning in the blip pile)

I'll play Intimidation as you suggest it is meant to be played !

Thanks.

Nope, you have to choose them all. This came up last night. The librarian removed the blip then intimidation went off.. DOH we were no longer traveling at that point. We would love to have chosen 0 instead of 5, but that didn't make sense. This brave marine goes to scare off the stealers, realizes he doesn't really want to now and says, "Just kidding."

in fact, as all the ability of the marines are optional, you can consider the intimidation card as a simple "move+activate".

if you choose to actually intimidate, and roll a 4, you cannot choose to send back to the blip only 1 GS, for sure.

but before rolling the intimidation die, you can't choose not to roll it.

that's the way I understand the rules.