As you might know,alphabets in hindi are divided on the basis of there phonetic properties.There are 5 vargs (Groups) and nov-varg consonants.Each varg in turn consists of 5 consonants.
like: 1st varg is ' क ख ग घ ङ ' etc.
Moving to your question, diffrence lies in pronounciation -
1. त - as in 'waTAshi'
2. थ - as in 'THAnks'
3. ट - as in 'TOmaTO'
4. ठ - as in 'kaTHmandu'
त and थ belong to same varg [4th] i.e, there pronunciation is similar but when used in different words they give rise to entirely different meanings e.g. 'taali' means clap but 'thaali' means a plate!.
ट and ठ belong to same varg [3rd]. Difference is similar as above.Example could be 'टिकाना' is to make something stable n still while 'ठिकाना' would means someones address.
Let me know if my explanation makes any sense!