Q:

Multiple SQL Server installations on ONE server? 

Across our business we have several low-volume SQL Server installations; added as we have developed/acquired new products for the business.  Each one is located on a virtual server and for argument sake let's say there are 5 such servers and each has been allocated 8Gb RAM and 4 CPUs ... in my mind, I think that it would make more sense to collated these onto ONE server, beefed up with memory/CPU. 
I see the PROs of this being that it makes it easier to keep track of the servers, to maintain them, upgrade (when required), assigning permissions would be easier and the performance may be increased (by adding additional RAM/CPU).  Presumably, we could also have the server replicated/load balanced to give additional resilience. 
Am I correct to think that? 
What would be the considerations AGAINST doing this? 

A:

To assist with your research, this is often called "Instance Stacking" when discussing installing multiple instances of SQL Server on the same box. 
You can read about some of the pros and cons here: Should I Install Multiple Instances of SQL Server? 
The biggest downsides would be maintenance / availability, which Iłya Bursov mentioned in the comments:

if this one server is down - all applications are down too (for example during upgrade)

Over time, it can become very difficult to schedule downtime as more apps are added to the machine. 
The other tough one is performance.   Making sure that the max and min memory settings for each instance or configured to keep from oversubscribing the box.   Going all rocket surgeon with affinity masking to make sure CPU is distributed (or not doing that and letting one instance trash the server). 
This also makes it tough to understand wait stats and other performance metrics, since they are usually reported at the instance level.   You'll need to aggregate them somewhere, or correlate the stats between the different instances. 

You can weigh these challenges against the licensing savings / convenience of only having one Windows box.   It sounds like perhaps these applications are not super resource intensive, or not 24/7 or business critical, in which case you could be in for a quick win in terms of licensing and manageability!   But it's good to see all sides in order to make an informed choice.