I have a Lumia 930 running the latest slow branch tech preview and I recently upgraded my data plan from 3G to 4G/LTE. But now the phone is consuming battery like crazy even when I'm not really using it that much during the day. So I wonder If I could force it to use 3G and if I could switch to 4G again when I need it (eg. When watching Youtube or Netflix etc.) To reduce this ridiculous battery drain?
Yes, this setting an be set by going to Settings -> Network & Wireless -> Mobile & Sim
From there, tap the link for SIM settings you'll then see an option to choose the fastest connection speed to use (which, on my phone, gives the options of 3G or 4G)
Go to settings>Network & Wireless >Mobile & Sim >SIM Settings >Mobile network> choose Highest Connection Speed(2G,3G or 4G).
You can switch between 3G and 4G in this way, I don't know whether there is an app for this.