This would indeed be the IP addressed assigned to the VLAN itself
VLANs do not really have IP addresses assigned to them. They have a network assigned to them, or a subnet, or a network range, however you want to refer to it. The address the OP supplied us is an assignable address within the range of 192.168.4.1-255. So lets say the range is applied to a group of servers so on a Cisco switch and we give the VLAN a description of "Server VLAN", 4.100 would be an address that can be given to an individual server. When referring to the Server VLAN, generally one may use the VLAN number or the network address, but typically not a specific address and the whole mask. At least the network admins I work with do not.
As I mentioned above, the OPs address can be a gateway address, but typically would not be because when you think of an environment like a large corporation, if you do not have a system of how gateway addresses are assigned, keeping track of them can be rather difficult. Thus most network admins use the first or last assignable address of a given range for the gateway. In the case of the OP, that would be 192.168.4.1 or 192.168.4.254. I'm not saying this is always the case, rather best practice and generally makes the most sense.
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.
This statement is confusing to me. We don't know anything about the address the OP gave us except the range it exists in, because the OP never said on what device it was found. We do not know if it is the address of a switch, a server, an AP, a computer, a printer, etc. So how you would know that from the small post from the OP wrote is beyond me.
I agree it doesn't have to be the gateway and I have already mentioned this. As I already explained, when you look at most large companies (but this is Cisco's best practice and is usually applied to most businesses) you actually find that gateway addresses will be the last or first assignable address in a range. 4.100 would be in the middle and would make no sense to be a gateway address. While some network admins might assign it that way, keeping track of this would be cumbersome, especially in increasing network sizes. This becomes even more true when HSRP and such technologies are used which take up two address on each layer 3 interface and give out a third address for the gateway. Keeping track of hundreds of such gateways when HSRP is being used becomes very difficult if there isn't a system for assigning addresses. Think of a company that might have 100 different VLANs...