If I had 2 cards and added one or two cards  by eliminating
an opponent giving me only3 or 4 but a set, can I cash in cards or do I need to
have a minimum of 5 to do this?

Yes you need to have 5 cards or more to be able to cash cards just after eliminating an opponent.

Yes, you need to have 5+ cards for a mid-turn card turn in after eliminating a player.  Some games have the option of returning to the placement phase after you've attacked, but I imagine the game you're playing does not that that setting.

I win Ed, I was faster ;)

There are, of course, many caveats to this on this site.  What Ed and Toto stated is true for the standard RISK game, and many boards use the standard rules, but many more do not.

Things that can effect this:

Return to placement from attack: with this on you can turn in cards anytime.

Card capture on/off: if off this is a moot point

Reserve Units/elimination bonus: If there is an elimination bonus, but reserves are off you are returned to the placement phase.  From here you can turn in cards no matter how many you have. (At least I think so - maybe return to attack from placement needs to be on also.  This one regularly confuses me.)

Maximum cards allowed: If more than 5 then that number is what determines things.

And there may be something I am forgetting.  In any case I regularly review the rules of most boards I play since its hard to keep them all straight.

Amidon37 wrote:

Reserve Units/elimination bonus: If there is an elimination bonus, but reserves are off you are returned to the placement phase.  From here you can turn in cards no matter how many you have. (At least I think so - maybe return to attack from placement needs to be on also.  This one regularly confuses me.)

Also, even if there is reserve allowed, if the elimination bonus would put you over the reserve you are returned to the placement phase.