hi,
some things to consider:
- depending on your access type, i.e. delivery protocol, there are huge difference in terms of processing power requirements. e.g. pppoe may used by your ISP and various implementations of pppoe stack differently scales. see next point.
- opnsense as i know still based on FreeBSD (if not please ignore), and pppoe in particular is not scaling well in FreeBSD as it is a single threaded app. or at least was the last time i used.
- to route 4Gbps up/down you need a massive hardware. a plastic router will not fit for sure. even a x86 will sweat because they miss the special network chipset from enterprise grade real routers. real routers are expensive not just because vendors go for profit.
- unless you are up to something not typical home usage I cannot imagine how 4Gbps can be utilized by a household in reality.
- delivering 4Gbps also a challenge from port type point of view so there might be an ISP marketing BS involved here. my ISP sell too 2Gbps ... on paper but in practice it is a 2 x 1Gbps connection. so for 4Gbps you would need a 10G nic to receive ISP connection then either you distribute it via 1G switch or need to invest in 10G switch and 10G client nics ... very-very expensive but it is your pocket. in former case your clients will receive max 1Gbps anyhow. (note: there is 2,5G and 5G but as i see they even less available then 10G).
- having X bw from ISP is basically covering speed of the wire but does not say / guarantee anything about ISP to Internet speed or speed between you and your selected service provider (i.e. Steam/Netflix/etc download will not be quicker as there is speed limit on their side, and through all the way involving many other parties from them to you). so should not expect that suddenly "internet" will be also 4Gbps up/down for you.
in short: if you can afford, sure go ahead and buy all things on 10G and set your expectation upfront.
edit: read this pls So you have 500Mbps-1Gbps fiber and need a router READ THIS FIRST