It wasn't a singleplayer or multiplayer problem, it was an AI issue. It still is. Do you really think the 4 per player limit matters to the AI on a small map where a player maybe occupies six or seven planets? I tmakes no difference to them, they wouldn't have built more than those 4 in the first place.
On bigger maps you're likely have multiple enemies at once, meaning if anything it makes the issue worse by multiple AIs spamming the nukes on YOU (yes the AI still gangs up on humans even if they are not allied with each other) with no chance of retaliation. After all, you only have four nukes, and they have 4x9 (worst case), in clean numbers 36 superweapons moving about. Way to shoot yourself in the knee.
As I see it now there are two ways of handling the superweapons:
1. Remove the frickin limit, make a lever in startup screen to deactivate them (and fix the delivarence. Won't ever get tired of noting this). The way supers are now they are more of support powers than anything else. Players can trip over them once or twice, but that's it...
2. Limit supers to 1 per faction and make them actually SUPER. Including all the benefits and backdraws. You have to sink tons of money into them. When they are completed they have zero AM and have to charge up for 10 to 15 minutes, also big fat warning for all players ("our scientists noticed a giant source of energy charging up" or whatever). If the player manages to shoot they have to affect multiple planets at once, with the center planet (the one you aimed at) getting the worst of it. The superweapons are supposed to force players into action, break stalemates or tip the balance of battle. If people freak when hearing the superweapon announcement GREAT, that's what they are suppposed to do.