I walked this stage last year and was very pleased with my decision. Be aware, it can be very cold at the height of the stage. The previous day (end April) had been warm and sunny but when we reached Fuenfria we were walking through low cloud and I have never felt so chilled in my life. Be prepared.
San Ildefonso is a charming town and the palace well worth a visit. The fountain display was great fun. We then had a short 12km walk into Segovia where we dumped our packs at Hostal Duerma Vela for a charge of 3 euros (
www.duermevelahostel.com, tel
691 282 445 /
921 047 004) and were free to see the wonderful sights the city has to offer unencumbered.
Later in the day we took the river walk to Zamarramala where the albergue is large, clean and very comfortable.
You can see my blog post of this stage
here
My wikiloc record of the stage is incomplete, something went wrong at the beginning of the recording, but you can see most of the route at the link below...
https://www.wikiloc.com/wikiloc/view.do?id=19077329 (stage 4 - Cercedilla to La Granja de San Ildefonso)
I can't help with accommodation in Medina de Rioseco as I was lucky enough to stay at the convent, but it is a sizeable town and I imagine there are various options.
Buen camino!