Can I increase signal strength on my Samsung Galaxy s4 active?
When connected to LTE, the phone is also separately connected to the GSM network for voice. Voice does not go over LTE so switching your data to WCDMA (HSPA) won't make any difference in the quality of voice calls, unless you are using VOIP calling. 

There isn't much you can do to boost the signal other than not using a case or if you do, stick to a thin TPU case that is just colored (ie no carbon fiber or chrome or wood or metal). If it is at your home, you may be able to see if your carrier offers a network extender. The network extender plugs into your home Internet and emulates a small cell tower in your house.