Replacing what works with like kind is always a solid plan.
Lets take a look at other options though.
The current system has a quantity of heat being added to a quantity of water. A simple way to look at reducing the total number is to add fewer, larger heaters.
Larger ones take up more space, generally it will be a more efficient use of space. If the space they go in is constrained in a way that won't accomodate the larger size however, that's a problem.
Fewer larger ones will likely be more efficient in terms of energy usage, but you give up redundancy/spare capacity. A good system is oversized a little, so right now with one down your tenants likely don't notice a difference. If you change the system to have 3 larger units now when one goes down you're much more likely to be under sized (you lose 25% of your capacity with 4, 33% with 3).
Lets say you have 4, 80 gallon heaters. That's 320 gallons. It is possible that there are controls that maximize this usage, completely using up one water heater and then switching to another, but unlikely. You could invest in such a control system and drop to two 80 gallon heaters, reducing your operating costs by half, but incurring some expense with the controls. You have more complications here, and fewer people will be able to work on your system. Not as many youtube videos to tell you how to fix it.
So these permutations continue. There's a lot of ways to do this, and depending on the specific details there definitely is a right answer that would minimize your costs long term.
But nobody on the internet is going to be able to help you. You'll need an expert to do a site visit. And that's going to be some up front cost. It will likely be worth it, and I'd be surprised if it couldn't pay for itself in 5 or 10 years. But. It might not.
Replacing what works with like kind is a solid plan.