Amos 4:4 & 5

"'Go to Bethel and sin; go to Gilgal and sin yet more. Bring your sacrifices every morning, your tithes every three years. Burn leavened bread as a thank offering and brag about your freewill offerings - boast about them, you Israelites, for this is what you love to do,' declares the Sovereign Lord." NIV translation

Bethel speaks of their place of worship while Gilgal speaks to their place of remembrance (see Joshua 4). They were going through their religious motions which was bad enough but they had the stones of Gilgal to remind them of the truth. It was bad enough to be going through the motions but to forget what God had done on their behalf was even worse.

Amos 4:6

"'I gave you empty stomachs in every city and lack of bread in every town yet you have not returned to me,' declares the Lord." NIV translation

God had brought drought and famine to the people to jog their memory about Him and the fact that He is Jehovah-Jireh their provider. Even in desperate times, they did not return to the Lord.

Amos 4:7 & 8

"'I also withheld rain from you when the harvest was still three months away. I sent rain on one town but withheld it from another. One field had rain; another had none and dried up. People staggered from town to town for water but did not get enough to drink, yet you have not returned to me,' declares the Lord." NIV translation

The Lord explains that He gave them extreme examples (one field would receive rain while the next would not) so that it could not be blamed on natural forces. The same type of thing is going to happen just before the tribulation described in Revelation. At that time, one man will be taken to be with the Lord while the one standing beside him is left to deal with the trials that are to come. This, too, is in hopes that those who are left will turn to the Lord and be saved.