Go to Settings on your device, choose Accounts and find Google Account. If you still have not added one, now is the good time to do that.
Then click on your account and tick the option "Contacts". It should now be synchronized with the Google server. You should repeat the same actions on the other device - that way the old phone will send your contacts and the new one will receive them.
Yes, Redmi 4A supports OTG function. If you are trying to connect pen drive please ensure that the file system is FAT 32. But it has a file size limit of 4GB only. You can check the complete specifications here.
You can also check this Video for detailed information:
You can find the full specs here: Redmi 4A
Samsung Z4 vs Xiaomi Redmi 4A
Both the phones have their pros and cons, I think you should go for Xiaomi Redmi 4A. Xiaomi Redmi 4A has the following advantages over Samsung Z4: - 1. Xiaomi Redmi 4A has 5.0" display size while Samsung Z4 has 4.5" display size. 2. Xiaomi Redmi 4A has 2GB RAM while Samsung Z4 has 1GB RAM. It is not like that it is a one sided game, Samsung Z4 has the following advantages - 1. Samsung Z4 has 1.5GHz Processor while Xiaomi Redmi 4A has 1.4GHz Processor.
Verdict - In my opinion, Xiaomi Redmi 4A is a better phone in overall performance but if you need a phone with good front camera for taking selfies, you can go for Samsung Z4.
You can also compare these devices on 91mobiles. Link -Samsung Z4 vs Xiaomi Redmi 4A