You mean in general?

They are probably the most used engine. I don't see any reason to avoid them, unless you needed more power.

When it comes to reliability it depends on many factors, but the key one of course is the manufacturer:  besides assembly, you have to design in reliability from the beginning.  Let’s assume for sake of discussion it is naturally aspirated.  In general they will last longer and have a lower cost of maintenance/repairs than an engine with more cylinders and/or V-configuration (V6, V8).  Also, if the I4 is very strained in that vehicle application then longevity will definitely be impacted, so it’s not always clearcut.  The best I4’s in terms of reliability (relative to other manufacturers) comes from Toyota.

Reliable? That depends on the make and model.

Lowet cost of maintenance? For inline 4x Usually yes, because it has less complex than V configured engines. 

Typically yes. I mean, to be honest a 4-cylinder engine is really one of the simplest motors you can get. All the high strung exotic cars with V12's, turbo V8's and V10's are a pain in the neck to work on. But most maintenance on a 4-banger can be done in the driveway and it is pretty hard to make a mistake. Parts are also usually cheap and available.

less parts, more simple a 4 cyl motor should absolutely be more reliable than a 6 or an 8 cyl. My wife’s 2002 Corolla had 525k before she sold it. That 4 banger was still wanting to go.