@<a href="http://www.runnersworld.co.uk/localiser/member.asp?MemNo=649245"><span style="color: #0066cc;">El Wendigo</a> I'm not an expert, but we have fairly similar times, goals, races.
I ran Barcelona Marató on 25 March 12 in 4h 49mins the temperature was about 18 degrees celsius, 5 weeks later............................................I also ran MK 2012 in 4h 32 mins, 3 hundreth of a second slower than my previous Pb at the time set in VLM 2010.
2 weeks after MK 2012 I ran Prague 2012 (13th May) and set new Pb of 4h 27mins temperature was 13 degrees celsius.
You can run the full distance in training and you can use marathon races as training for the race you think your more likely to achieve your sub 4h 30 goal.
If you run better in warmer temperatures that's great for Barcelona , otherwise incorporate running in heat into your training schedule.