Actually, current advice these days for patients with bad backs is to keep active, obviously within moderation.
Loose rivets is correct in stating that disc pressures are greatest when seated but remember also that it is reduced in standing and even less when lying down. However, "Lots of rest....horizontal" is a prelude to rigor mortis ! From what you have reported (4 weeks, back at work, able to drive etc) then keep moving if you can. Avoid postures / activities / tasks that aggravate the symptoms. Anything that eases the symptoms should be encouraged.
Most physios (certainly Australia, New Zealand and Uk) are highly trained and some only specialise in musculoskeletal conditions. Furthermore, the days of traction as the only treatment or as described by Loose rivets are long gone.
It sounds as though your physio is on the right track. A MRI can give peace of mind in terms of what may be going on. However, if you want to get it sorted you need to think positively about looking after your back. A good book to read is "Treat your own back" by Robin McKenzie, a great New Zealand physio and a pilot (PPL) if my memory serves me correctly. Take on board whats in this book and add to it some core stability training because current research shows that recurrence rates are reduced.
Move well and stay well.