Backpack sprayers have many advantages. First and foremost, the ability to carry up to 4 gallons will allow you to finish most jobs in one pass. Secondly, the backpack sprayer allows you to reach areas that would normally be difficult to spray.  A handheld garden sprayer is not easy to haul and does not hold as much product.  One of the drawbacks to hauling the backpack sprayer around is that the weight of the pack is not well supported by the straps. Most models have thin straps that dig into a person’s shoulders when the tank is full, and the back of the tank is not padded, which can be uncomfortable for some people.  Another problem with the sprayer is that the tank tends to leak. There are a few ways that you can make modifications are to create new straps and to alter the seal to be more effective.
Make It Comfortable
If you are going to be hauling around a backpack, it makes sense to make it as comfortable as possible, right?  You can do this by adding padding to the back and shoulder supports.  Wider straps are more comfortable and should be utilized.  A quick way to modify your 4 gallon sprayer is to use a hiking backpack which have metal supports on the back that you will be able to attach the sprayer tank to.  You can screw the tank right on to the support brackets using waterproofing measures for the screws.  The tank will still be on you back but with the added comfort of a waist strap and padded shoulders and back.
Alternative Solution
You can also modify a backpack to be attached to the sprayer.  Different patterns are available as well as being able to purchase a ready-made kit.
Stop The Splashing
By modifying the backpack sprayer’s harness, much of the leaking should stop or be significantly slowed down, as the modification will help the tank shift less during the applicator’s maneuvering. Another problem with some tanks is that the lids will leak.  One way to slow this down is to try and make sure that the seal is in good condition; A dry or hardened seal will need to be replaced. You can also get rid of rough edges on the cap and tank to better allow for direct contact. Using sandpaper; smooth out rough plastic edges a little at a time.  It is possible to do more harm than good so just round off the edges, do not diminish them.
Preventive Maintenance
It is possible to avoid sprayer leakage problems with maintenance.  Use petroleum jelly on seals and rinse out the sprayer and nozzles after each use.  It is not recommended that you store your chemical in your sprayer; chemicals can cause reactions to different plastic and metal parts making them wear faster than they might normally.
Hopefully this makes your experiences with 4-gallon backpack sprayers more enjoyable!