Connecting batteries in parallel to the existing batteries that are connected in series
I've decyphered and redrawn your, well, let's call it a graphical intimation, as a schematic, to make it clearer why this won't work



simulate this circuit &ndash; Schematic created using CircuitLab
Now it's obvious that F's connections are short-circuiting C and D, and E's connections are short-circuiting A and B.
If you merely disconnect the shorting connections as another answer has suggested, then you'll end up with two high capacity batteries in series with two low capacity batteries. Do Not Do This The capacity of the string will be no more than the single batteries in series. If you monitor the total voltage on the string for endpoint, you'll overdischarge the single cells, and the whole series string will be impossible to charge properly.
We assume that the series batteries are of the same capacity, type and age, in order to properly monitor, charge and balance them. If they are not, do not connect them in series.
The best way to use your two extra batteries would be to sell them, and use the money for something else.
Another way, if they are fairly new, would be to buy two extra batteries so you have two, 48 V strings. If the batteries have any appreciable service, do not do this, as connecting batteries of different age in series runs into the same monitoring/charging/balancing problems as batteries of different capacity in series.
Another way is to cycle them in and out of your service string of 4, so that wear and tear on them will be reduced. This needs regular changing and attention to routine, so may be difficult. There's wear and tear on the connections as well.
Another way to use your two extra batteries is to connect them to a 24 to 48 V boost converter, and use them instead of the series 4 for part of the load cycle, or in parallel with them if your boost converter can tolerate that. If your booster is current output, like a LED driver for instance, then it can easily be paralleled with the 48 V string, when the total load current is above the output current of the booster.
Yet another way is to configure them into two 36 V strings, or a single 72 V string, and use a DC-DC converter to bring them to your desired 48 V output. You gain voltage stability and flexibility with this method, which might be beneficial.