I don't have a P4, but I'd imagine the log would have battery data. Is it a CSV file? I hope so. You can look directly at the log with a text editor, but better yet, look at it with Dashware, because you'll see it in a spreadsheet format.
From the opening screen:
File/Edit Data Profiles (you can't edit them, but you can look at them)
Click the Open Data File button and find the log file and click once on it, then click the Open button at the bottom of the screen. The file should appear on your screen, but it will be unformatted.
Click on the tiny white/grey down arrow in the upper right corner, and choose an appropriate viewer (I use Flytrex).
Next click the Test File/View Data button. You should now see a new screen with your file formatted.
I'd imagine if you peruse the top of the columns, you'll find all sorts of battery info such as voltage, temperature, cell info, serial number, product date, etc.
Now that you know where in the file the battery data is located, you can make your own gauge to show the data on your screen. If you don't know how to make your own gauge, you can use one of the many pre-made gauges and simply redefine the data it reads so that it will show the battery data you wish to display.
Here's an example of a video from last winter showing overlayed battery data (percentage, voltage, temp) taken from both a Flytrex log and a Litchi log (the battery temp starts at 28C because I pre-warmed it prior to the flight):
Edit: Actually the above was a waypoints mission and it flew out of Litchi range so it's not a good example. This one might be better but doesn't have battery temp - I only had my Phantom for a month at this point last fall.