Oh , 
4 months - poor baby.
Is your other pig the same age?
I've had two occasions in the last year where I've had a lone boar.
They both fretted, it's normal for them to feel bereft and bereaved, but I'll share with you what we id to make it (hopefully) easier all round.
First, now your pig has died, the remaining pig needs to be given time to grieve. He needs to know his cagemate has died , so don't sneak the body away, leave it with him, just tucked away in the bedding, and let them have some alone time.
Tomorrow you can remove him, before he gets stinky. If you bury him, seal him up well, (our GP2 was dug up by foxes, luckily he was in a sealed toiletry bag)
Give your remaining pig a few days to grieve, keep him indoors if you can (important to keep an eye on him if his cagemate had an infection, I'd take him to the vet as a precaution for a check up)
To find him a new mate, contact some Rescues, they will have a young boar piglet and will 'boar match' them.
If you want sows, you'll need to arrange to neuter your boy.
You need to make everything neutral and disinfect before your new pig comes home.
I used a brand new paddling pool to 'bond' ours, then they slept in my DD room in the cage before they went out to the Pighouse.
We had originally GP1 and GP2, two rescue brothers. GP2 died a year ago ( they can go downhill very quickly, he was fine, then doing that tucked up look). He went to his guinea-pig vet, nothing definitive wring, got antibiotics but died that night. GP1 was with him, DD and I were cuddling them.
We left him together with GP1, then we kept GP1 in DD room at night.
After 6 days, along came GP3, 6 week rescue boar.
GP3 is a big, cheeky Rex who quickly installed himself as Alpha Pig (GP1 was 'okay, you be in charge'
GP1 died about a month ago ,so we had GP3 (he weighs 3lb) who I didn't want to boarmatch because he used to bonk GP1 . (GP1 was ' ahem d'you MIND ' and walked away, but I didn't want to risk a piglet with this over enthuiastic boar 
)
So, (same Rescue as GP3) along came GP4 and GP5 - two 10 month old sows who had been pg and their piglets weaned.
GP3 has been neutered, just over a week till he's sterile. (Girls are separate but side-by-side to chat)
Then we need to boar-date them.
I use a steamer and vinegar to clean then sprayed with Vikron S to clean and disinfect. One of the saddest things is eradicating all the trace of your guinea-pig.
We gave lots of cuddles and human chat (Radio left on for him) while he was alone. But GP1 and GP3 were definately quieter alone. (He's found his squeak again )
Boar piglets are easier, I'm planning how I'm going to introduce my boar to the sows. They are skitty (I did ask for feisty, I got feisty ) not particularly well socialised but today I held GP5 and thought , she's made huge inroads in 3 weeks.
This is a bit of an essay, well done if you got this far - anything else, give a shout out.
for your piggie, , GP1 (The Mighty Chester) will guide him across The Rainbow Bridge.