daniel97 might be confusing Virgin's Pendolino bicycle policy, which is 4 bicycles OR 2 bicycles and 1 tandem.
There are 4 spaces PER Virgin Voyager (all of which are reservable on single Voyagers), in a publicly accessible area behind the driver's cab in Coach A / 'G' (the original Bombardier layout, which is much preferable to Arriva's botched "refurbishment"). On a double Voyager serving two separate destinations with a split or merge (1), all 8 spaces can be reserved.* But where a secondary Voyager is detached from or attached to the rear of a primary Voyager mid-way through one journey (2), the rail industry's reservation software is unable to identify the 4 additional spaces in the additional Voyager. So there is no way to reserve these spaces, but you can have a look if there are any available spaces when the train turns up. To further confuse matters, where a secondary Voyager is detached from or attached to the front of a primary Voyager mid-way through one journey (3), all 8 spaces can be reserved again!*
An example of (1) is the 18.10 from Euston to Holyhead and Wrexham, splitting at Chester. 4 spaces can be reserved in the Holyhead unit and 4 in the Wrexham unit, even if making a journey wholly before the split point - e.g. Euston to Nuneaton.
An example of (2) is the 19.10 from Euston to Holyhead. This is a double Voyager from Euston, with the rear unit detaching at Chester. The rear unit is 'dead' for reservation purposes. It isn't even possible to make seat reservations in this unit (which is a nuisance, given that the 19.10 is the first off-peak service to Milton Keynes / Nuneaton / Stafford / Crewe / Chester). When the train operates, the reservation displays above the seats in this unit are black / unlit. Another example of (2) is the 18.23 from Euston to Shrewsbury. This is a double Voyager from Euston, with the rear unit detaching at Wolverhampton.
An example of (3) is the 05.49 from Glasgow to Euston, via Wolverhampton. Until Wolverhampton, this is a single Voyager, operating as Coaches A to E. At Wolverhampton, it is attached to the rear of a secondary Voyager already waiting on the platform (in other words, the secondary Voyager is attached to the front of the primary Voyager). From Wolverhampton, there are 8 reservable bicycle spaces and seats are reservable in all 10 carriages.
Where a double Voyager serves the same destinations throughout, seats are reservable in both units, but bicycle reservations can't be made in the rear unit. I'm not sure why this is.
Yes, it's confusing!
*If attempting to reserve e.g. 6 spaces, you'll need to make one reservation request for 4 spaces and another reservation request for 2 spaces, because a single reservation request can't be split across units.