I need to but a hole in some tongue and groove boarding for a backbox for an electrical socket. The boarding is the usual 4" pine stuff.
I was thinking of using something like this - Erbauer DRA55458 Square Shank Multi-Material Drill Saw Set 3 Pcs - Screwfix
Dumb idea or not?
Sounds like an ideal time to buy a new tool!... 🤣
https://www.screwfix.com/p/erbauer-emt300-qc-300w-electric-multi-tool-220-240v/622fx
They look like they'd make a horrible mess.
If you dilligaf, crack on, otherwise yeah, a multitool as above sounds ideal.
once you've got a multitool in your arsenal, you'll wonder how you ever coped without one. 18v is worth the extra over corded as they're so useful in quick jobs.
depends on the hole (size/shape) you want - personally i wouldnt recommend the first link, it wont give much accuracy, think of it as a file on steroids, the latter is expensive/overkill unless you will have other uses for a multi-tool
They look like they'd make a horrible mess.
How so? I get that it'll make a load of dust (but I can have MrsIHN standing next to me with the hoover which'll get the worst of it as I'm cutting) and it might not be the neatest cut, but that'll be covered by the lip of the backbox and the socket itself.
Hire one
If you want to make a round hole an oval one, those saw bits are OK. They're not OK for cutting any distance. They're just a bit rubbish. Drill a hole in one corner of the location (with a normal drill bit) you want then use a pad saw to cut the rest of the square out. It'll be neater, faster and about the same cost as buying a set of those drills.
the latter is expensive/overkill unless you will have other uses for a multi-tool
...you'll find uses for one of those - I guarantee it!!! Cheaper options are available! 🤣
4" pine T&G will be quite thin.
It depends what you have already, but either a tenon saw or padsaw will do it.
Start the tenon saw at a 45 angle with short strokes until you get going, start the padsaw from a corner hole.
12" hacksaw blade with a duct tape handle will work if that's all you have
Multi-tool is preferable IME
Multitool is good and eternally useful but for this job I love this little dude
Quick, slightly dangerous, depth settable so no risk of nicking a pipe or wire. May seem expensive but these are a "one job" tool so try ebay, I bought mine for £30 and I'll sell it for £30 when I'm done with the refurb. Only downside is you can't go right to skirtings because of the shape, you always end up breaking or multitooling something. But still.
Oh, too late for an edit, you can use an angle grinder with a thin blade but it'll absolutely annihilate the blade, and obviously there's more cut-through risk. Wear serious face protection if you do, metal blades hate cutting wood
Drill a hole in one corner of the location (with a normal drill bit) you want then use a pad saw to cut the rest of the square out. It'll be neater, faster and about the same cost as buying a set of those drills.
This. When I read the OP, I was thinking 'why would you do anything other than this? Am I missing something?'
*Obviously drill the hole on the *INSIDE* of the opening you want to make.
If you just want something that's capable of making an untidy hole in wood, I can lend you our guy Sprocket for the weekend, he can also remodel your garden, provide an alternative warning system to, say, a Ring doorbell, and shred cardboard boxes, with or without contents.
Drill a hole, use a padsaw, or sploog out on a multicutter.
Those drill bits things look like a disaster about to happen.
The dill bits look like hard work and potentially difficult to make a neat job.
cheapest way, four holes one in each corner cut from hole to hole with a pad saw.
Best way, multi tool with decent wood cutting blade.
Er....hire one
If the wall behind is anything other than plasterboard, I can't see those drill saw things working.
Beg or borrow a multi-tool saw, it will be done in minutes and tidy. Give it back with a new blade if you blunt it on the plaster/bricks behind 🙂
I should have read the OP's post properly. Thought he wanted a large hole (for cables) and not a recess for a socket. Must get myself one of these multi-tools - no idea what they do, but they sound useful ! LOL Moretools.com
Late to the thread but a wooden hammer and sharp chisel is what I'd try first.
Floorboard saw.
Its going to be easier to work with and leave a considerably better finish. 
Although I have a whole bunch of tools, last week I needed to make a 36mm circular hole in some (nice quality)18mm MFC. None of my hole cutters was just right so I used the old stitch drilling technique (lots of 4mm holes next to each other on the cut line) and it worked a treat, because the toilet flush it was for has an overlapping flange. It wasn't neat but it didn't have to be.
So, I did it using a 5mm drill bit to make a row of overlapping holes and a pruning saw. Sorted.