The Defence 1 and Defence 4 share a name, but they occupy different places in a sleep kit.
The Defence 1 is the lighter choice for warmer conditions. It suits trips where a small packed load is important and overnight temperatures remain within its published comfort range. It can also form part of a layered system when the components are compatible and there is enough room to preserve loft.
The Defence 4 carries more insulation for colder nights. That additional protection comes with more weight and packed volume, so it is not automatically the better choice for every trip.
Compare four things before ordering: comfort temperature, bag dimensions, packed size and weight. Then add the variables that no product table can settle for you. A cold sleeper may need more margin. A poorly insulated mat can undermine either bag. Wind, moisture and an exposed bivy site also change how the system performs.
For mixed-season use, avoid buying to the coldest imaginable scenario if most trips are warm. A modular combination may be more comfortable and easier to pack across the year. For regular cold-weather use, starting with the Defence 4 is usually the more direct option.
Always confirm the current model specification and available length or colour before purchase.