I have TIR 3 PRO w VE,(I too have window about 4' directly behind me)and trust me that as well is light sensitive..my advice is to buy some quality (thick) curtain's to block out the light if u have a window behind ya ;)..or just wait until the sun goes down-or untill it clouds over :)
p.s if i were buying from new I would personally go for TIR 4 PRO..coz I'm so impressed with my TIR 3 PRO (which i havn't had for long)..I wouldnt fly without it