Actually you don't need an Agbay leveler to get your cakes the correct height. You just need a basic ruler and an idea of how much filling and icing you are going to add in. This is going to be long but I promise it's not that big of a deal. Just write it down the first time you do it and use that for future reference.
Let's say you bake cakes in 2 inch pans. For simplicity let's asssume you are not torting your cake. Just 2 layers of cake, icing in the middle and on top.
If you want to be consistent with your filling between the layers use either a tip or an open coupler to put the icing between the two layers. Your icer tip is fine if that's the amount you like to use. I use an open coupler if I'm not torting. That means my inside icing is just under 1/2 inch once it's all compressed. My top icing is going to be a little bit less than that generally so let's say 1/4 of inch. So on average I loose 3/4 of inch to icing and filling them my total cake height should be 3.25 inches, so I would need to level each of my cake layers to 1.625 inches (don't be that exact. I'd probably go for 1.5 inches each.
You don't have an Agbay, so the question is how do I get them to 1.5 inches every time. After your cakes are cool, take several cake circles (one size down from the cake size) and stack them up to 1/2 an inch. Tape them together and wrap them in something like Press and Seal. This will let you reuse them later.
Place the cake circles in the bottom of the pan and place your cooled cake on top. You cake is now sticking out 1/2 inch from the top of the pan. Using a long serrated knife, level your cake using the top of the pan as your guide. You now have 1.5 inch layer cake. Now repeat this with the other tier, fill and ice and you should be right at 4 inches.
One good tool to invest in, is a sewing ruler. These are the little rulers that have a slide on them. (You can get them at walmart). If you're afraid your tiers are to too tall or too short, once you have them frosted but not decorated or covered in fondant, check the height with a sewing rule. You can add frosting to adjust.
If you're doing fondant don't forget to allow for fondant height to be included in your calculation. Everyone stresses about this but if you can get it pretty close to 4 inches (a tiny bit over or under is fine) then it's not an issue. If there is a big gap between then it's a pain. But using the sewing ruler you should know before you put in the legs.