JavaScript (ES6),  118  113 bytes

(w,h,M,F)=>(g=(c,n)=>'01210'.replace(/./g,i=>c(+i).repeat([F,M,n][i])))(y=>g(x=>'###+X#++'[y+x*5&7]+' ',w)+`

`,h)

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Commented

(w, h, M, F) => (       // given the 4 input variables

  g = (                 // g = helper function taking:

    c,                  //   c = callback function returning a string to repeat

    n                   //   n = number of times the painting part must be repeated

  ) =>                  //

    '01210'             // string describing the picture structure, with:

    .replace(           //   0 = frame, 1 = matte, 2 = painting

      /./g,             // for each character in the above string:

      i =>              //   i = identifier of the current area

        c(+i)           //   invoke the callback function

        .repeat         //   and repeat it ...

        ([F, M, n][i])  //   ... either F, M or n times

    )                   // end of replace()

)(                      // outer call to g:

  y =>                  //   callback function taking y:

    g(                  //     inner call to g:

      x =>              //       callback function taking x:

        '###+X#++'      //         figure out which character to use

        [y + x * 5 & 7] //         by applying a small hash function to (x, y)

        + ' ',          //         append a space

      w                 //       repeat the painting part w times

    )                   //     end of inner call

    + 'n',             //     append a line feed

  h                     //   repeat the painting part h times

)                       // end of outer call

Python 2, 98 bytes

w,h,a,b=input()

a*='+'

b*='#'

for c in b+a+h*'X'+a+b:print' '.join(min(c,d)for d in b+a+w*'X'+a+b)

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Prints a space-separated grid, strictly following the spec. I'm amused that *= is used to convert a and b from numbers to strings.

Python 3 can save some bytes by avoiding ' '.join, maybe more by using f-strings and assignment expressions.

Python 3, 95 bytes

def f(w,h,a,b):

 a*='+';b*='#'

 for c in b+a+h*'X'+a+b:print(*[min(c,d)for d in b+a+w*'X'+a+b])

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Charcoal, 48 bytes

ＮθＮηＮζＮεＵＯ⁺θ⊗⁺ζε⁺η⊗⁺ζε#Ｍε↘ＵＯ⁺θ⊗ζ⁺η⊗ζ+Ｍζ↘ＵＯθηXＵＥ¹

Try it online! Link is to verbose version of code. Explanation:

ＮθＮηＮζＮε

Input the four values.

ＵＯ⁺θ⊗⁺ζε⁺η⊗⁺ζε#

Draw the framing.

Ｍε↘ＵＯ⁺θ⊗ζ⁺η⊗ζ+

Move to and draw the matting.

Ｍζ↘ＵＯθηX

Move to and draw the painting.

ＵＥ¹

Double-space the output horizontally.

Alternative solution, also 48 bytes:

ＮθＮηＮζＮεＵＯ⁺θ⁺ζε⁺η⁺ζε#ＵＯ⁺θζ⁺ηζ+ＵＯθηX‖ＯＯ←θ‖ＯＯ↑ηＵＥ¹

Try it online! Link is to verbose version of code. Explanation:

ＮθＮηＮζＮε

Input the four values.

ＵＯ⁺θ⁺ζε⁺η⁺ζε#

Draw the framing, but not to the left or above the painting.

ＵＯ⁺θζ⁺ηζ+

Draw the matting, but not to the left or above the painting.

ＵＯθηX

Draw the painting.

‖ＯＯ←θ‖ＯＯ↑ηＵＥ¹

Reflect and double-space the output horizontally.

Python 3.8 (pre-release), 116 115 bytes

lambda a,b,c,d,e='#',f='+':"n".join((g:=[e*(a+2*c+2*d)]*d+[(h:=e*d)+f*(a+c*2)+h]*c)+[h+f*c+'X'*a+f*c+h]*b+g[::-1])

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First attempt at golfing, will be improved soon.
a is width, b is height, c is matte width, and d is frame width.

-1 bytes using the := operator to define h as e * d

EXPLANATION:

lambda a,b,c,d,e='#',f='+':          Define a lambda which takes in arguments a, b, c, and d (The width of the painting, the height of the painting, the padding of the matte, and the padding of the frame width, respectively). It also defines variables e and f as '#' and '+', respectively.

    "n".join(                       Turn the list into a string, where each element is separated by newlines

        (g:=                         Define g as (while still evaling the lists)...

            [e*(a+2*c+2*d)]*d+       Form the top rows (the ones filled with hashtags)

            [(h:=e*d)+f*(a+c*2)+h]*c Form the middle-top rows (uses := to golf this section)

        )+

        [h+f*c+'X'*a+f*c+h]*b+       Form the middle row

        g[::-1]                      Uses g to golf the code (forms the entire middle-bottom-to-bottom)

    )

Javascript, 158 bytes

(w,h,m,f)=>(q="repeat",(z=("#"[q](w+2*(m+f)))+`

`)[q](f))+(x=((e="#"[q](f))+(r="+"[q](m))+(t="+"[q](w))+r+e+`

`)[q](m))+(e+r+"X"[q](w)+r+e+`

`)[q](h)+x+z)

Can probably be trimmed down a little bit

f=



(w,h,m,f)=>(q="repeat",(z=("# "[q](w+2*(m+f))+`

`)[q](f))+(x=((e="# "[q](f))+(r="+ "[q](m))+(t="+ "[q](w))+r+e+`

`)[q](m))+(e+r+"X "[q](w)+r+e+`

`)[q](h)+x+z)



console.log(f(3,2,1,2))

Wolfram Language (Mathematica), 152 bytes

(t=(p=Table)["# ",(x=2#4+2#3)+#2,x+#];p[t[[i,j]]="+ ",{j,z=#4+1,#4+2#3+#},{i,z,x-#4+#2}];p[t[[i,j]]="X ",{j,#3+z,#3+#4+#},{i,#3+z,#3+#4+#2}];""<>#&/@t)&

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Perl 6, 98 bytes

{map(&min,[X] map (($_='#'x$^d~'+'x$^c)~'X'x*~.flip).comb,$^a,$^b).rotor($b+2*($c+$d)).join("n")}

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This is a port of xnor's Python answer.

Perl 6, 115 bytes

->a,b,c,d{$_=['#'xx$!*2+a]xx($!=c+d)*2+b;.[d..^*-d;d..^a+$!+c]='+'xx*;.[$!..^*-$!;$!..^a+$!]='X'xx*;.join("

")}

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Roughly golfed anonymous codeblock utilising Perl 6's multi-dimensional list assignment. For example, @a[1;2] = 'X'; will assign 'X' to the element with index 2 from the list with index 1, and @a[1,2,3;3,4,5]='X'xx 9; will replace all the elements with indexes 3,4,5 of the lists with indexes 1,2,3 with 'X'.

Explanation:

First, we initialise the list as a a+2*(c+d) by b+2*(c+d) rectangle of #s.

$_=['#'xx$!*2+a]xx($!=c+d)*2+a;

State:

# # # # # # # # #

# # # # # # # # #

# # # # # # # # #

# # # # # # # # #

# # # # # # # # #

# # # # # # # # #

# # # # # # # # #

# # # # # # # # #

Then we assign the inner rectangle of +s

.[d..^*-d;d..^a+$!+c]='+'xx*;

State:

# # # # # # # # #

# # # # # # # # #

# # + + + + + # #

# # + + + + + # #

# # + + + + + # #

# # + + + + + # #

# # # # # # # # #

# # # # # # # # #

Finally, the innermost rectangle of Xs.

.[$!..^*-$!;$!..^a+$!]='X'xx*;

# # # # # # # # #

# # # # # # # # #

# # + + + + + # #

# # + X X X + # #

# # + X X X + # #

# # + + + + + # #

# # # # # # # # #

# # # # # # # # #