Hello, I am missing a few weapons in worms 4 mayhem, that i was used to playing with in worms 3d. The weapons i really missed are:

-Sticky Bomb
-Uzi
-Mortar
-Petrol Bomb
-Viking Axe

The full list on wikipedia lists much more, also weapons who werent in worms 3d:
Mortar, Homing Pigeon, Red Bull, Ming Vase, Uzi, Petrol Bomb, Blowpipe, Mega Mine, Sticky Bomb, Viking Axe, Mad Cow, Lottery Strike, Doctors Strike, Sheep Strike, Teleport, Binoculars, Freeze, Scales Of Justice, Earthquake, Nuclear Bomb, Laser Sight, Armageddon, Bananette, Dragon Ball, Flame Thrower, Kamikaze, Laser, Magic Bullet, Mine Strike, Minigun, Mortar Bomblet, Napalm Strike, Super Banana Bomb, Invisibility, Sheep Launcher, Battle Axe, Handgun, Long Bow, Suicide Bomber, Aqua Sheep, Mole Bomb, Mail Strike, Mole Squadron, Blow Torch, Pneumatic Drill, Bungee, Sally Army, MB Bomb, Skunk, Mike's Carpet Bomb, Homing Cluster Bomb, Homing Air Strike, Cloned Sheep, Postal Strike

Is there a way i can get the weapons that i had in worms 3d into worms 4? (preferably without replacing any weapons in worms 4, but just adding them)

Those weapons were not programmed into the game, so it's impossible to add them to it. They were likely removed from the game due to them having their purpose done better and more efficiently by other weapons in the game or they just didn't work well with the 3D format.

That, or they didn't work as well with Worms 4's maps, which are generally far larger than in Worms 3D.

Are you really missing them? Most of them I found rather unuseful, for the same reason Junkman mentioned: They just didn't fit well into 3D Worms.

But I thought it was possible to add weapons by tweaking? Sorry if I'm wrong, I don't know alot of this method.

I know you can replace weapons with your own, and I think it might be possible to add weapons to the game. Somewhere on youtube, actually I think I saw someone add a weapon to the weapon panel. (filled in one of the blank spaces)

You can pick all the weapons you listed in a website of russians whose admin is Alexbond.
But, as the people before me said, you have to replace the original weapons of worms 4 mayhem in order to get them functionally. Moreover, you can't play online with those unless another player has the same, identical files and weapons.

Is there suposed to be empty slots on the weapon panel? I usually find them all full in the older worms games, in the new 3d format do they make the weapons panel free spaces on purpose? I just want all the weapons possible in both Worms 3D and 4 but it seems almost impossible to get them.