I have the same 4 output hi tech charger for my first quad, Q500+. That's a great charger for dumb batteries, but is risky with P4. You definitely don't want to use the equalizer connections since each battery has its own equalizer. The P3 could be charged ok if you use just the hi amp charger lines, but P4 connectors have several parallel charge lines, you'd have to make custom cable adapters, if you could find the connectors, and you have to know what each connection is. A good tech could figure it out with a slew of measurement tests of the oem charger, but if you don't know what you doing, YES, you're asking for trouble. Let us know if you burn the house down [emoji12]
I like my triple charger/discharger that I bought from DJI, that's what I'd recommend.
What I'd like to know is.... when your friend charges his P3 batteries, do they get a full charge? Those hitech charge lines have a monitoring system, I'm not sure if they are compatible with DJIs input voltage requirements on the battery charge connector.