I had 3 puffers living together happy for about 4-5 months. Than i discovered that it's 2 males and possibly female(no line on belly). So i decided to buy one more female, I got her earlier this week. But I noticed that 3 'old' puffers sleeping together and new puffer sleeps somewhere I can't even sometime find her.

Just wondering, if they will get use to her or it was a bad idea to get tankmate?

Depends on their personalities, how much space they have, and how you arrange the tank. Moving all the decorations when adding a new puffer can be helpful if there is violence.

The tank is about 10g big and heavy planted. Unfortunately have no idea if they aggressive to each other or not coz as you know when you come close to tank they will stop what they been doing will give all their attention to you so i can really tell what they are doing when i am not there

I'd suggest a 20 gallon long tank for the best chance for this to work out. Maximizes territory, and 3 DPs in a ten gallon is already totally maxed out. Alternately, 29 gallon tanks have the same footprint as 20 longs, but are taller, and give you even more water to dilute wastes, which will lead to longer fish life.

I was going to put them all in my new 50G tank, just waiting for this tank to be empty - selling all fish from it. The reason I got last puffer was I live in New Zealand and puffers are extremely rare here, last time I saw at my local fish store more than 6 months ago.
Also I notice they looks ok to each other, don't bite, chase or anything. They sleep all together now in same plant every night

As they grow, aggression will probably increase.

Yea, you'll want them in that 50 gallon asap, having more space will make them reaching full adult habits much easier on them.

Finally, my 50G tank is free. Puffers are so happy in their new home. They are so funny, I have been watching them the other day and noticed that they swimming all together in one line for like 15 min. Than in next second you look at the tank and it's feels like it's empty - everyone is hiding.
Also they seems to sleep in one place all 4 of them, really interesting. I think it could be because this place might have low water flow. I have Fluval 203, AquaOne power head and BluePlanet UV filter in tank, water is always crystal clear.

Will post pic later today.

Sounds like an awesome DP home, I wish I had a 50g full of DPs. I am excited for pics of it.

Finally got pic of new home. I have now 6 puffers:3m and 3f. They all seems really happy and only harassing otocinclus They coming really close to him and pretending they going to swim pass in next second they nipped his tail fin. I have also 4 otocinclus in tank - 2 of them for some reasons only coming out at night, and 2 others always swimming around.

Unfortunately I have algae problem in tank so my plants looking really bad right now and I lost so far 2 beautiful red plants

Anyway a pic of 50G tank: 6 dp and 4otocinclus