Q:

Do i need a fan, or will ventilation holes suffice? 

this setup has not yet been completed.  I'm only asking so I can order a fan if needed
I plan on connecting the following elements directly to my pi4-

relay(i think I'll need two)
small LED with resistor
Piezo element
power pack, via modified usb c cord(using 4-5 of these batteries)

all of this will be in as small a container as i can find and squeeze it into, which will also be shared with these items, not powered directly from the pi-

power pack for EL wire(2 AAs)
power pack for bright LED(1-2 AAs or 3 AAAs)

I may add more later on, but for now, this is all I need to squeeze into a box.  My question is- is it fine just to drill some holes in the box for ventilation?  Or do I need to attach a fan to keep it from overheating? 

A:

That will depend on things that you've not told us.  But not to worry, because we'd have a hard time answering your question with any authority even if you had! 
That sounds nonsensical, but here's what we know, and why a definitive answer is difficult:

The RPi 4 has what you might think of as a closed-loop thermal management system that is "baked in". 

The firmware that implements this thermal management system is closed-source, and details of its implementation are not disclosed. 

What we do know from testing and what's been published by "The Organization" is that as component temperatures rise, the thermal management system acts to "throttle" performance.  In other words, the thermal management system will not allow the RPi to "melt down" or self-destruct. 

And that brings us to this reasoning:

Your need for fans and heat sinks may be determined only through experience and testing the RPi 4 in the environment in which it will operate. 

"Environment" includes primarily ambient temperature and processor workload. 

Put your RPi 4 in its enclosure, and run your application(s) in an ambient temperature close to what it will see in the field. 

Monitor the performance and temperatures of the RPi 4.  If the RPi 4 cannot perform the calculations/processing needed in the allotted time, OR if the temperatures are higher than you'd like: Add fans/heatsinks/ventilation accordingly.  If you're satisfied with the performance and component temperatures, you need do nothing further - the system will manage this for you. 

Some references:
RPi 4 Thermal Teting is mostly self-congratulatory publicity, but you will pick up some information.  For example: RPi will run cooler if placed in a vertical orientation. 
Independent RPi 4 Thermal Testing by Tom's Hardware provides a slightly better explanation

