You can totally get a huge tolerance without a physical dependency- but my experience is you needa be chipping pretty seriously to get much of a tolerance...By which I mean using 2-3 days a week (which is abig part of why people rarely chip with 'done) with 1-3 day breaks, the occasional 5-10 days clean with an equal number of 2-7 day binges that result in minor 'walking' withdrawal.
I'm with the others- it sounds like your shit is getting watered down hardcore. 400mg is a serious junkie dose- that's the sort of dose I'd take, but I am a junkie and have one of the highest tolerances of the people I know...So something isn't right. Either you're downplaying your use, the drugs are cut to hell or you're some kind of freakatron.
If they're pills or you're 100% sure it's not being watered-down then you must have a fairly high baseline tolerance (which isn't unheard of- for every person who's allergic there's a hardhead) and/or a large bodymass. It shouldn't take more than a month to significantly reduce your tolerance, maybe three to get it as low as it's gonna go.
EDIT: Maybe you have the dose wrong- does your country have multiple concentrations of methadone solution on the market? Maybe you use to get 20mg/1ml stuff while now you're getting 5mg/ml stuff- so say you use to get 20mg/ml 20ml would be 400mg, but now you're getting 5mg/ml so 20ml is a far more reasonable 100mg...I mean, that'd be totally possible where I'm from.