The Philadelphia Realty Transfer Tax is 4.1%.3.1% (City) + 1% (Commonwealth) = 4.1% (Total)The tax rate is based on the sale price or assessed value of the property, plus any assumed debt.

I have heard that now in the great Commonwealth of Massachusetts, 4-unit buildings are treated more or less like 5+ when it comes to building codes, meaning that I must hire a GC and architect to draw up plans for this minor renovation.

Doing either requires you to sign a form stating you are a Virginia resident, and the Commonwealth's Department of Taxation takes that as you stating an intention to remain in Virginia after your military service is done.

But I’m going to guess that the Commonwealth of Pennsylvania and their attorneys have more time and money than you when it comes to litigation. 

Eric Lesser – Bill S.1553 An Act Relative to the Regulation and Taxation of Transient Accommodations in the Commonwealth

https://www.ziprealty.com/property/320-COMMONWEALTH-AVE-_UNIT_42-BOSTON-MA-02115/36227507/detail?

That said, I believe I read some posts on BP suggesting that the Commonwealth is becoming more vigilant about this issue. 

Hello, I am a long time reader of BP and am in school at Virginia Commonwealth University, studying real estate.

Housing and Urban Development today announced HUD will speed federal disaster assistance to the Commonwealth of Puerto Rico and provide support to homeowners and low-income renters forced from their homes due to Hurricane Irma.Yesterday, President Trump issued a major disaster declaration for Culebra and Vieques municipalities.

I recommend PBMares or Commonwealth Business Services.