I have a Ampeg Rocket Bass RB-115 combo amp. Its 200w but only does 100 without another cabinet.
I found a local deal on a Ampeg SVT-410HLF 4x10 500w cabinet rated at 4ohm.
On my Rb115 it says to connect an 8ohm speaker.
Is this going to blow up my amp if i connect these together?
most probably, yes
Combining an 8 ohm and a 4 ohm cab will produce a 2.67 ohm load. Unless your RB-115 will accept a 2.67 ohm load or lower (2 ohms) then yes, you could very well damage the amp.
My guess is it will not handle a load below 4 ohms which combining it with another 8 ohm speaker will provide. Search for another 8 ohm cab if you want to get the full 200w from your combo.
I’ll add to this that while there are amps that can operate with a 2 ohm load they are typically not those found in combo amps. They will almost always be amp heads that will run 2 ohms so they can run virtually any speaker combination from 8 ohms down to 2 ohm.
Genzler and Mesa are two that come to mind an IIRC some Ampeg heads as well. There may also be others that I’m not familiar with never having used them whereas these three I have.
Yes, most amp heads that can handle 2 ohm seem to be in the $1000+ range 
You should be able to pick up a used Genzler Magellan 350 for half that or maybe a bit less. It can handle a 2.67 ohm load . They’re still not inexpensive but unless you play large venues without a FOH PA you’ll probably never need more than that for the rest of your days.
But then you’d also need to buy cabs so for now your best bet would be to just buy an 8 ohm cab. Ampeg has the VB-115 and VB-112 neo 8 ohm cabs that would work with your combo. There’s also the Heritage and PortaFlex series cabs which are a little less money. There should be used ones of those around as well.
I think you’ll be surprised at how much more output you’ll get by adding another 115 or 112 cab and allowing the amp to push it’s full 200w.
I’m fine for now… Maybe I’ll add another DG112N someday 
Thanks everyone. I went and picked up that 4x10 stack, got it from a local guy who picks up storage units and resells the stuff for $200. Was a little skeptical but after seeing how many buildings he has on site and multiple employees just listing/selling the things he picks up it seemed pretty apparent hes just trying to move it out and make room for more. He had been dropping the price but nobody was biting. Its in great shape.
Ill use the money I saved on this and get a amp head in the future.