When I was young (mid-80's), I used to play with terrific toys and I
cannot remember the name of these "kind of alien soldiers" with a
hologram on their chest that was representing one of the fourth
primary elements (earth, wind, fire (lol) and water). They had a
strong smell of plastic and I am not sure whether their chest hologram
was always shown. May be it was only appearing with heat... I was a
French little boy so I do not know if these toys were sold worldwide
but I guess so.

(sorry for the translation mistakes...)

Dear pierrelouis-ga,

I believe that you are referring to Battle Beasts.

Here are some descriptions from web sites which match yours. All have
further information and pictures.

?Battle Beasts came out in the late 80's and like most toys, had a
gimmick to help sales. Each Battle Beast had a heat sensitive sticker
on his chest which, when rubbed would reveal the warrior's strength.
The symbols would represent either fire, wood or water and could be
used in a rock, paper, scissors type game?fire beat wood, which beat
water, which beat fire.?
http://en.wikipedia.org/wiki/Battle_Beasts

?Battle Beasts were small 1" tall figures released by Hasbro in 1986.
Each Beast had heat sensitive sticker on their chest. When rubbed, it
would reveal if the Battle Beast was Fire, Wood or Water. Each Beast
also carried their own weapon.?
http://www.toyarchive.com/BattleBeasts/BattleBeasts1.html

?This time it is taken up by the Battle Beasts, fierce creatures that
are half-robot and half-beast. Each beast has the power of one of the
three elements, Wood, Water, and Fire. Who will triumph in this
elemental battle of beast versus beast??
http://www.virtualtoychest.com/battlebeasts/battlebeasts.html

Postings on message boards,

?Without a shadow of a doubt... BATTLE BEASTS!!!!!!!!
They were so damn cool and I had like 200 of them. They're probably
still all in a box in my parents' basement. The whole gimmick of the
bodyheat-sensitive hologram on their chest that would turn to Fire,
Water, Wind, or Earth was awesome. I remember playing with those damn
things for hours!!?
Google cache only 
http://72.14.203.104/search?q=cache:aWeW2sXkfQsJ:forums.comicbookresources.com/archive/index.php/t-57076.html+toy++hologram+chest+earth+wind&hl=en&gl=uk&ct=clnk&cd=3

?There was those Battle Beasts with the heat sensitive stickers on the
front. You picked you fighter, gave it a rub and looked to see if you
beat your opponent.
Fire burnt Wood.
Wood floated on Water.
Water put out Fire.?
http://forum.rpg.net/archive/index.php/t-66308.html

I hope this answers your question. If I have incorrectly identified
the toys, or the answer is unclear, then please ask for clarification
of this research before rating the answer. I shall respond to the
clarification request as soon as I receive it.

Do not be concerned about your translation, it was excellent and gave
me all the information I required.
Thank you
answerfinder

Search strategy
toy  hologram chest earth wind, led me to 
"battle beasts"
://www.google.com/search?hl=en&lr=&q=%22battle+beasts%22&btnG=Search

Exactly what I was looking for. I cannot believe there are so many
posts on "Battle Beasts"...