"It's needed we have up to 4 area or can i use only single area (only area 0)."
Possibly. This is an "it depends" kind of question. Your attachment only shows 8 routers, and "cores", so it's unknown what the complete topology might be. Topology matters for this question.
Also, although you have 4 geographic network regions (?), your area choices extend beyond just one area or four. You might also have two or three areas. Again, much would depend on the overall topology.
BTW, a (very old) OSPF rule, I recall, being no more than 3 areas per router. If your area zero routers were the ABRs, they would violate that rule, if you had the four areas as shown in your attachment.
Another (very old) rule of thumb in no more than 50 routers per OSPF area, but many have reported having single areas more without issue. Of course, realize, there's the RFC OSPF and Cisco's OSPF, the latter Cisco has done much to make it work "better" (at least handling some of the common pitfalls of OSPF).
"What is advantages and disadvantages using muti and single area?"
Multiple areas allow a much larger OSPF AS, that's the advantage. The disadvantage is multiple areas are more complex. (Some of multiple area OSPF issues can sneak up on you if you don't fully understand multiple area design considerations. [Heck, about a year ago, a posting had a query about a problem with OSPF multi-area routing - which was impacted by whether routers were using an older or newer RFC - Cisco supports both, but had different defaults, I recalled, based on platform!])
The Achille's heel of OSPF has been the Dijkstra's algorithm, which runs per area. The problem with this algorithm, it doesn't scale linearly. I.e. double the size of the topology, it take much more than double the time to compute.
A major "gotcha" of traditional OSPF is ANY change in the area topology requires re-computation for the whole area topology, but among other Cisco enhancements, their iSPF feature, can, in some situations, short circuit much SPF computation. Multiple areas, though, is a way to bound SPF computation time.
Much has be mentioned in other replies about reducing the resource requirements of LSA databases by having multiple areas. Well that's true, but perhaps not emphasized enough, to really leverage multiple areas, you need to use stub areas and/or address summarization toward area zero. (BTW, besides reducing LSA database resource needs, stubs and/or summarization can reduce route table resource needs too.)
"If every router on each site have different WAN speed example 30Mbps and 70Mbps, how the traffic flow will utilize the bandwidth. Is WAN with 70Mbps will have more traffic or both WAN only can have 30Mbps to load balance so WAN with 70Mbps will under utilize?"
OSPF only provides ECMP (equal cost multiple path). If OSPF "sees" the 30 and 70 Mbps costed the same, it would use both paths equally. If it "sees" one path with a lower cost, it would use that path alone.
Cisco's OSPF implementation auto costs based on bandwidth, using a default base metric of 100 Mbps being considered the most possible bandwidth. So, in a default Cisco OSPF implementations, likely only the 70 Mbps WAN link would be used.
