Hi OPIE,
I agree, you'll be ok for the 4 mile trip home, just get a 4-7 wire pin adaptor.
However, I would recommend a brake controller if you plan to do any traveling out of state. Each state has different towing laws wrt when trailer brakes are required. Check out this
State Towing Laws for more information on each state. MA has a 10,000 lb limit until brakes are required

, but most states are 3000 lbs, inc. NY, VT, CT (NH is 1,500 lb limit). That is if the trailer weighs more than 3000 lbs, it needs to have brakes. So if you travel out of MA to one of your neighboring states, you will need a brake controller. Unless you want to try the argument that the law only states that the trailer has brakes; and the trailer does have brakes, The law says nothing about requiring them to be hooked up.
If you’re handy, it’s a job you can do yourself. Since you only have a 4-wire plug now, you’ll probably have to run the wires from the front to the back plug receptacle. Depending on your tow vehicle, this could be easy or very difficult. I remember years ago when we had a 7-wire plug installed on our Ford Aerostar, the wires were run inside and plug just hung out the rear tail gate.
Sorry I don't know the price of installation. I use Camping World a lot as a price bench mark for compaison shopping. CW has a $79 installation charge when you buy a brake controller from them. You can use this price for comparison.
Hope this helps.
Hutch