@Thomas Lee
Yes! I can help! Killeen fourplexes are my thing.
The $600 and $500 are both low - the 4-plexes asking $200k+ in 76549 should go for $550/2 bed and $650/3 bed meaning $2400 gross for your typical 3/2/2/3 building (some buildings in that area are 2/2/2/2 or 3/2/2/2 - so keep that in mind when shopping!).
There is a new fourplex listing on Sylvia that pretty closely meets my ideal metrics in a fourplex asking $218k. I haven't checked on it in a couple days and who knows, might already be under contract. Multi-families are going more quickly. Inventory is about half what it was even just a year ago.
I wrote an article here recently about what to expect in the Fort Hood market - hopefully that helps!
https://www.biggerpockets.com/blogs/10185/66309-ki...
I'm not as sophisticated as coming up with IRRs, but a 6-7% cap rate is realistically what you can expect from an area fourplex, yes, if getting it from the MLS (which you likely will be - few distressed sellers and I don't run into many off market multi-family owners anymore).
Your vacancy should be better on average than 10%, but I would stick with those assumptions just to be conservative.
Fourplexes are going to have better returns than duplexes. Duplexes are more saturated with owner-occupant/retail buyers, and the prices are driven higher. That said, there are some pros. I wrote an article about that on my own website recently as well - what to look for in duplex v fourplex v SFH.