I have seen people in Terraria with gerbil pets, I really want one, but I don't know how to get pets.
Please help.
Terraria – get pets from
terraria
Related Solutions
As of the latest version of Terraria on the PC, it is now possible to change your hairstyle in-game through the Stylist NPC who was introduced in 1.2.3.
The Stylist, like the Goblin Tinkerer and the Angler, must be found and freed before she can provide her service. She can be found in the Spider Nest mini biome, found in worlds generated in version 1.2 and above.
For the cost of 1 
she will give you a hairstyle that could have been chosen at character creation, and for 5 she will give you a hairstyle that is exclusive to her. In addition, she can change your hair color for 1 and sells special hair dyes that have a wide variety of interesting effects for 5 - 15 .
The game has no way to obtain a jump height of 4 blocks, you can see all the possible jump heights in the game in this infographic from reddit user DataNalle.
I would try reinstalling the game from scratch. So long as you backup your save data (located in Documents\My Games\Terraria\ on Windows and Library/Application Support/Terraria on Mac.) you should be fine reinstall/verify your game.
Verify game cache: Right click game on library list > Properties > Local Files > Verify Integrity of Game Cache
Delete local content: Right click game on library list > Delete Local Content...
If none of this works and your not completely adverse to cheating you can use http://yal.cc/r/terrasavr/ to give yourself a jump boost buff/item which my help.
Best Answer
There are currently 26 pets available for the PC version of Terraria. You can find a pet by getting them as a drop from successfully defeating certain bosses or finding them in certain chests. The Terraria Wiki covers Pets more thoroughly, I recommend you check it out.