Why does a VLAN have an IP address?
Solution 1:
A VLAN doesn't have an IP address. It's the devices communicating on the VLAN which have IP addresses
Solution 2:
I believe this is merely an IP address for the VLAN-aware device that happens to be on that VLAN. You need it if the device in question is going to be doing routing involving that VLAN, or if you expect to manage the device (ie through SNMP) on that VLAN.
(after your update) I see you are enabling routing through the Netgear switch this VLAN is defined on. This IP is the address that local systems will use as the router.
Solution 3:
You can think of a VLAN as a network if that helps. So where I work we have all kinds of networks, ones for PLCs, Servers, Networking Devices, Wireless, etc. To make things easy with our networking environment we created VLANs for each one of these. This way we can have multiple devices from each main category connected to any switch we want but we can have that device in it's own VLAN.
In other words, without VLANs you would need a lot more networking devices because each access layer switch would have to be on it's own network, just like your home network. Any switch(es) you plug into your router or modem at home is going to be on the same network. With VLANs we can plug a server, PLC, regular user, and wireless AP into the same switch if needed and still have them all be in their respective VLANs.
VLANs themselves do not have IPs assigned to them like computers and servers do. Instead they are assigned networks. Which is why many people exchange the word VLAN for network and vice versa. So at home your probably have a 192.168.0.0 255.255.255.0 network or something similar. Well for VLANs we do the same thing...
We might assign 192.168.1.0 255.255.255.0 to servers, 192.168.2.0 255.255.255.0 to PLCs, 192.168.3.0 255.255.255.0 to regular users, and 192.168.4.0 255.255.255.0 to wireless. This means a server will get a different IP than a PLC, etc. Thus our segregation with VLANs. And if you understand how IPs and Subnet masks work, you will know that each VLAN above will have its own network and communication and be separate from each other.
Your IP you mentioned above, 192.168.4.100 255.255.255.0, is likely an IP within a VLAN and not a gateway for that VLAN or anything like that. Typically the gateway is the first or last usable IP in the network range but it doesn't have to be. The broadcast address is always the last IP in the range and can never be assigned to anything, in your case the broadcast IP would be 192.168.4.255.
I hope this ramble helps...
Solution 4:
From the wikipedia (since it says it well):
Virtual LANs are essentially Layer 2 constructs, compared with IP subnets which are Layer 3 constructs. In an environment employing VLANs, a one-to-one relationship often exists between VLANs and IP subnets, although it is possible to have multiple subnets on one VLAN or have one subnet spread across multiple VLANs. Virtual LANs and IP subnets provide independent Layer 2 and Layer 3 constructs that map to one another and this correspondence is useful during the network design process.
The IP address the OP listed as:
ip address 192.168.4.100 255.255.255.0
This would indeed be the IP addressed assigned to the VLAN itself. Specifically, it is the IP address of the "switch" the VLAN is on. It doesn't necessarily have to be the gateway IP for the VLAN but typically is since you typically setup IP addresses on the VLAN at the Layer 3 "router" for the VLAN and thus use this IP address for the gateway for clients on that VLAN. The Layer3 switch will have IP routing/forwarding enabled if necessary.
The mask basically says that the VLAN is the 192.168.4.0/24 network. It's up to you to decide if you want to actually use the 192.168.4.100 as the actual client gateway IP or if this is simply a management IP for the switch/vlan.
Solution 5:
For most Level 3 switches out there, the line you specified represents the gateway address of the VLAN.
Don't confuse it with default gateway, which only applies when routing is turned OFF.