Is this safe to mod with?

[Vapor Vinny]

I've built several mods using 4AAA batteries. I know they're safe. But what about this as a holder? I don't know what they mean by "serial".

$1.09 4*AAA Serial Cylindric Plastic Battery Holder - 4S1P at FastTech - Worldwide Free Shipping

 

[Chip_]

I've built several mods using 4AAA batteries. I know they're safe. But what about this as a holder? I don't know what they mean by "serial".

$1.09 4*AAA Serial Cylindric Plastic Battery Holder - 4S1P at FastTech - Worldwide Free Shipping

My little flashlight I bought from a dollar store, a couple of years ago, uses the battery holder similar to the one you show.
I would suspect that "Serial" means they are connected in "Series", in other words, the batteries are connected end to end, giving the highest possible voltage, i.e. 4*1.2V = 4.8V (sometimes full charge gives 5.3 volts).