Planning a cycling trip - any recommendations?

Coast to coast through Yorkshire, I did it a couple of years and had a good time, its also easy to add extra distance and there's some good hills in the moors and stuff.
Mostly quiet roads and good scenery (weather could be a bit iffy tho ).
The roads are not the smoothest at all times though.

Only 4 days would kinda limit you abroad. Is that 4 in total or 4 cycling days? For such a short period of time I'd stick to the UK.

Perthshire and the Angus Glens in Scotland would be grand, there's a lot of cycling friendly accommodation up that way because of the Etape, the roads are quiet and the scenery is beautiful. You would use either Pitlochry or Blairgowrie as a base.

I hear the English/Welsh border is fantastic too, always wanted to explore down that way.

Depends what you want. Climbs? Scenery? Quiet roads? Long days? Short days?

Here's some inspiration, home and away

Wales

Lake district

Majorca

Dartmoor Although probably not enough variety for so many days.

Also recommend Peak District, Exmoor, and Scotland (beautiful).

Why not wait for Paris-Brest-Paris? that's 4 days spent on the bike with no worries about accomodation.

With views like this...

How about NE Catalunya. You could fly into Girona or Barcelona and then head the region of Osona and La Garrotxa, an hour from BCN and 40 mins from Girona. Stunning scenery, smooth traffic free roads, lots of climbing. Better than the area around Girona, in my opinion, much more variety and less traffic.

Islay. Nice scenery, very few cars, relaxed atmosphere, and has the added benefit of there being several whisky distilleries.

Around Annecy (try bikeweekender in St Jean de Sixt), around Bourg D'Oisans or maybe down in the Pyrenees. Easyjet flights just cost me under £50 return to Geneva, though you need to factor in accommodation, bike hire/transport and airport transfers.